Early island-arc intrusive activity,Cordillera Central,Dominican Republic

Reconnaissance studies of early island-arc intrusions in the Cordillera Central of the Dominican Republic demonstrate that these rocks are mainly hornblende tonalite with lesser amounts of hornblende diorite, quartz diorite, granodiorite and quartz monzonite. Two plutons (El Bao, Medina) are petrographically and chemically homogeneous, whereas two others (El Rio and Loma de Cabrera) are compositionally heterogeneous. Samples from these intrusions range in SiO₂ from 49 to 70% with most rocks in the 59 to 62% range. K₂O ranges from 0.24 to 3% and averages 1.2%. Cu, Zn, Co, Ni, V and possibly Cr decrease with increasing SiO₂. Rb/Sr values for the intrusions are low but variable. Present-day ⁸⁷Sr/⁸⁶Sr values range from 0.7031 to 0.7045 for the El Bao and Loma de Cabrera batholiths and 0.7033 to 0.7091 for the Medina stock. These data do not generate isochrons. The Cordillera Central tonalite intrusions are the most abundant plutonic rock type in the Greater Antilles, although small, younger granodiorite and quartz monzonite stocks are present. The Cordillera Central intrusions are lower in SiO₂, K₂O, Rb, and Sr than the average composition of the Sierra Nevada batholith, but they are similar to the tonalites and trondjhemites from the western margin of the Sierra Nevada batholith. The low Rb/Sr ratios and low initital Sr⁸⁷/Sr⁸⁶ ratios for the Cordillera Central intrusions combined with the high liquidus temperatures required for the generation of tonalite magmas strongly favor a subcrustal source for these magmas in an island-arc setting.     

Magma accumulation rates and thermal histories of plutons of the Sierra Nevada batholith,CA

Zircon U–Pb geochronology results indicate that the John Muir Intrusive Suite of the central Sierra Nevada batholith, California, was assembled over a period of at least 12 Ma between 96 and 84 Ma. Bulk mineral thermochronology (U–Pb zircon and titanite, ⁴⁰Ar/³⁹Ar hornblende and biotite) of rocks from multiple plutons comprising the Muir suite indicates rapid cooling through titanite and hornblende closure following intrusion and subsequent slow cooling through biotite closure. Assembly of intrusive suites in the Sierra Nevada and elsewhere over millions of years favors growth by incremental intrusion. Estimated long-term pluton assembly rates for the John Muir Intrusive Suite are on the order of 0.001 km³ a⁻¹ which is inconsistent with the rapid magma fluxes that are necessary to form large-volume magma chambers capable of producing caldera-forming eruptions. If large shallow crustal magma chambers do not typically develop during assembly of large zoned intrusive suites, it is doubtful that the intrusive suites represent cumulates left behind following caldera-forming eruptions.     

Exposure of a late cretaceous layered mafic-felsic magma system in the central Sierra Nevada batholith,California

New U-Pb zircon ages for the Lamarck Granodiorite, associated synplutonic gabbro and diorite plutons, and two large mafic intrusive complexes that underlie them in the Sierra Nevada batholith are 92±1 Ma. These ages establish the Late Cretaceous as a period of extensive mafic-felsic magmatism in the central part of the batholith, and confirm the significance of mafic magmatism in the evolution of the voluminous silicic plutions in the Sierran arc. The lack of significant zircon inheritance in any of the units analyzed supports isotopic evidence that the Lamarck and other Late Cretaceous Sierran plutons were derived predominantly from young crust. Recognition of an extensive mafic-felsic magma system in the Sierra Nevada batholith emphasizes the importance of basaltic liquids in the evolution of continental crust in arc settings.     

Late Cretaceous-Paleocene Extensional Collapse and Disaggregation of the Southernmost Sierra Nevada Batholith

Geobarometric studies have documented that most of the metasedimentary wall rocks and plutons presently exposed in the southernmost Sierra Nevada batholith south of the Lake Isabella area were metamorphosed and emplaced at crustal levels significantly deeper (～15 to 30 km) than the batholithic rocks exposed to the north (depths of ～3 to 15 km). Field and geophysical studies have suggested that much of the southernmost part of the batholith is underlain along low-angle faults by the Rand Schist. The schist is composed mostly of metagraywacke that has been metamorphosed at relatively high pressures and moderate temperatures. NNW-trending compositional, age, and isotopic boundaries in the plutonic rocks of the central Sierra Nevada appear to be deflected westward in the southernmost part of the batholith. Based on these observations, in conjunction with the implicit assumption that the Sierra Nevada batholith formerly continued unbroken south of the Garlock fault, previous studies have inferred that the batholith was tectonically disrupted following its emplacement during the Cretaceous. Hypotheses to account for this disruption include intraplate oroctinal bending, W-vergent overthrusting, and gravitational collapse of overthickened crust. In this paper, new geologic data from the eastern Tehachapi Mountains, located adjacent to and north of the Garlock fault in the southernmost Sierra Nevada, are integrated with data from previous geologic studies in the region into a new view of the Late Cretaceous-Paleocene tectonic evolution of the region. The thesis of this paper is that part of the southernmost Sierra Nevada batholith was unroofed by extensional faulting in Late Cretaceous-Paleocene time. Unroofing occurred along a regional system of low-angle detachment faults. Remnants of the upper-plate rocks today are scattered across the southern Sierra Nevada region, from the Rand Mountains west to the San Emigdio Mountains, and across the San Andreas fault to the northern Salinian block.

Batholithic rocks in the upper plates of the Blackburn Canyon fault of the eastern Tehachapi Mountains, low-angle faults in the Rand Mountains and southeastern Sierra Nevada, and the Pastoria fault of the western Tehachapi Mountains are inferred to have been removed from a position structurally above rocks exposed in the southeastern Sierra Nevada and transported to their present locations along low-angle detachment faults. Some of the granitic and metamorphic rocks in the northern part of the Salinian block are suggested to have originated from a position structurally above deep-level rocks of the southwestern Sierra Nevada. The Paleocene-lower Eocene Goler Formation of the El Paso Mountains and the post-Late Cretaceous to pre-lower Miocene Witnet Formation in the southernmost Sierra Nevada are hypothesized to have been deposited in supradetachment basins that formed adjacent to some of the detachment faults.

Regional age constraints for this inferred tectonic unroofing and disaggregation of the southern Sierra Nevada batholith suggest that it occurred between ～90 to 85 Ma and ～55 to 50 Ma. Upper-plate rocks of the detachment system appear to have been rotated clockwise by as much as 90° based on differences in the orientation of foliation and contacts between inferred correlative hanging-wall and footwall rocks. Transport of the upper-plate rocks is proposed to have occurred in two stages. First, the upper crust in the southern Sierra Nevada extended in a south to southeast direction, and second, the allochthonous rocks were carried westward at the latitude of the Mojave Desert by a mechanism that may include W-vergent faulting and/or oroclinal bending. The Late Cretaceous NNW extension of the upper crust in the southernmost Sierra Nevada postulated in this study is similar to Late Cretaceous, generally NW-directed, crustal extension that has been recognized to the northeast in the Funeral, Panamint, and Inyo mountains by others. Extensional collapse of the upper crust in the southern Sierra Nevada batholith may be closely linked to the emplacement of Rand Schist beneath the batholith during Late Cretaceous time, as has been suggested in previous studies.     

The Garnet+Hornblende Isograd in Calcic Schists from an Andalusite-Type Regional Metamorphic Terrain, Panamint Mountains, California

Calcic schists in the andalusite-type regional metamorphic terrainin the Panamint Mountains, California, contain the low-varianceassemblage quartz+epidote+muscovite+biotite+calcic amphibole+chlorite+plagioclase+spheneat low grade. Near the sillimanite isograd, chlorite in thisassemblage is replaced by garnet. The low variance in many calcicschists allows the determination of the nature of the reactionthat resulted in the coexistence of garnet+hornblende. A graphicalanalysis of the mineral assemblages shows that the reactioncan not be of the form biotite+epidote+chlorite+plagioclase+quartz=garnet+hornblende+muscovite+sphene+H₂Obecause garnet+chlorite never coexisted during metamorphismand the chlorite-bearing and garnet-bearing phase volumes donot overlap. The compositions of the minerals show that withincreasing grade amphibole changed from actinolite to pargasitichornblende with no apparent miscibility gap, the partitioningof Fe and Mg between chlorite and hornblende changed from K_D(Mg/Fe, chl&amp) < 1 to K_D > 1, the partitioning betweenbiotite and hornblende changed from K_D (Mg/Fe, bio/amp) <1 in chlorite-zone samples to K_D > 1 in garnet + hornblende-zonesamples, and the transition to the garnet-bearing assemblageoccurred when the composition of plagioclase was between An₅₅and An₈₀. Both the graphical analysis and an analytical analysisof the compositions of the minerals using simplified componentsderived from the natural mineral compositions indicate thatat the garnet+hornblende isograd the composition of hornblendewas colinear with that of plagioclase and biotite, as projectedfrom quartz, epidote, muscovite, and H₂O. During progressivemetamorphism, chlorite+biotite+epidote+quartz continuously brokedown to form hornblende+muscovite+sphene until the degeneracywas reached. At that point, tie lines from hornblende couldextend to garnet without allowing garnet to coexist with chlorite.Thus, the garnet+hornblende isograd was established throughcontinuous reactions within the chlorite-grade assemblage ratherthan through a discontinuous reaction. In this type of isograd,the low-grade diagnostic assemblage occurs only in Mg-rich rocks;whereas the high-grade assemblage occurs only in Fe-rich rocks.This relation accounts for the restricted occurrence of garnet+hornblendeassemblage in low-pressure terrains. In Barrovian terrains,garnet+chlorite commonly occurs, and the first appearana ofgarnet+hornblende can simply result from the continuous shiftof the garnet+chlorite tie line to Mg-rich compositions.     

Experimental and geochemical evidence for derivation of the El Capitan Granite,California, by partial melting of hydrous gabbroic lower crust

Partial melting of mafic intrusions recently emplaced into the lower crust can produce voluminous silicic magmas with isotopic ratios similar to their mafic sources. Low-temperature (825 and 850°C) partial melts synthesized at 700 MPa in biotite-hornblende gabbros from the central Sierra Nevada batholith (Sisson et al. in Contrib Mineral Petrol 148:635–661, 2005) have major-element and modeled trace-element (REE, Rb, Ba, Sr, Th, U) compositions matching those of the Cretaceous El Capitan Granite, a prominent granite and silicic granodiorite pluton in the central part of the Sierra Nevada batholith (Yosemite, CA, USA) locally mingled with coeval, isotopically similar quartz diorite through gabbro intrusions (Ratajeski et al. in Geol Soc Am Bull 113:1486–1502, 2001). These results are evidence that the El Capitan Granite, and perhaps similar intrusions in the Sierra Nevada batholith with lithospheric-mantle-like isotopic values, were extracted from LILE-enriched, hydrous (hornblende-bearing) gabbroic rocks in the Sierran lower crust. Granitic partial melts derived by this process may also be silicic end members for mixing events leading to large-volume intermediate composition Sierran plutons such as the Cretaceous Lamarck Granodiorite. Voluminous gabbroic residues of partial melting may be lost to the mantle by their conversion to garnet-pyroxene assemblages during batholithic magmatic crustal thickening.     

A garnet–hornblende geothermometer: calibration, testing, and application to the Pelona Schist, Southern California

Abstract A garnet–hornblende Fe–Mg exchange geothermometer has been calibrated against the garnet–clinopyroxene geothermometer of Ellis & Green (1979) using data on coexisting garnet + hornblende + clinopyroxene in amphibolite and granulite facies metamorphic assemblages. Data for the Fe–Mg exchange reaction between garnet and hornblende have been fitted to the equation. In K_D=Δ (X_Ca,g) where K_D is the Fe–Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form: with very satisfactory agreement between garnet–hornblende and garnet–clinopyroxene temperatures. The thermometer is applicable below about 850°C to rocks with Mn-poor garnet and common hornblende of widely varying chemistry metamorphosed at low a_O2. Application of the garnet–hornblende geothermometer to Dalradian garnet amphibolites gives temperatures in good agreement with those predicted by pelite petrogenetic grids, ranging from 520°C for the lower garnet zone to 565–610°C for the staurolite to kyanite zones. These results suggest that systematic errors introduced by closure temperature problems in the application of the garnet–clinopyroxene geothermometer to the ‘calibration’data set are not serious. Application to ‘eclogitic’garnet amphibolites suggests that garnet and hornblende seldom attain Fe–Mg exchange equilibrium in these rocks. Quartzo-feldspathic and mafic schists of the Pelona Schist on Sierra Pelona, Southern California, were metamorphosed under high pressure greenschist, epidote–amphibolite and (oligoclase) amphibolite facies beneath the Vincent Thrust at pressures deduced to be 10±1 kbar using the phengite geobarometer, and 8–9kbar using the jadeite content of clinopyroxene in equilibrium with oligoclase and quartz. Application of the garnet–hornblende thermometer gives temperatures ranging from about 480°C at the garnet isograd through 570°C at the oligoclase isograd to a maximum of 620–650°C near the thrust. Inverted thermal gradients beneath the Vincent Thrust were in the range 170 to 250°C per km close to the thrust.     

Mantle derivation of Archean amphibole-bearing granitoid and associated mafic rocks: evidence from the southern Superior Province,Canada

Amphibole-bearing, Late Archean (2.73–2.68 Ga) granitoids of the southern Superior Province are examined to constrain processes of crustal development. The investigated plutons, which range from tonalite and diorite to monzodiorite, monzonite, and syenite, share textural, mineralogical and geochemical attributes suggesting a common origin as juvenile magmas. Despite variation in modal mineralogy, the plutons are geochemically characterized by normative quartz, high Al₂O₃ (> 15 wt%), Na-rich fractionation trends (mol Na₂O/K₂O >2), low to moderate Rb (generally<100 ppm), moderate to high Sr (200–1500 ppm), enriched light rare earth elements (LREE) (Ce_N generally 10–150), fractionated REE (Ce_N/Yb_N 8–30), Eu anomaly (Eu/Eu^*) 1, and decreasing REE with increasing SiO₂. The plutons all contain amphibole-rich, mafic-ultramafic rocks which occur as enclaves and igneous layers and as intrusive units which exhibit textures indicative of contemporaneous mafic and felsic magmatism. Mafic mineral assemblages include: hornblende + biotite in tonalites; augite + biotite ± orthopyroxene ± pargasitic hornblende or hornblende+biotite in dioritic to monzodioritic rocks; and aegirine-augite ± silicic edenite ± biotite in syenite to alkali granite. Discrete plagioclase and microcline grains are present in most of the suites, however, some of the syenitic rocks are hypersolvus granitoids and contain only perthite. Mafic-ultramafic rocks have REE and Y contents indicative of their formation as amphibole-rich cumulates from the associated granitoids. Some cumulate rocks have skeletal amphibole with X_Mg(Mg/(Mg+ Fe²⁺)) indicative of crystallization from more primitive liquids than the host granitoids. Geochemical variation in the granitoid suites is compatible with fractionation of amphibole together with subordinate plagioclase and, in some cases, mixing of fractionated and primitive magmas. Mafic to ultramafic units with magnesium-rich cumulus phases and primitive granitoids (mol MgO/ (MgO+0.9 FeO^TOTAL) from 0.60 to 0.70 and C_T >150 ppm) are comagmatic with the evolved granitoids and indicate that the suites are mantle-derived. Isotopic studies of Archean monzodioritic rocks have shown LREE enrichment and initial ¹⁴³Nd/¹⁴⁴Nd ratios indicating derivation from mantle sources enriched in large ion lithophile elements (LILE) shortly before melting. Mineral assemblages record lower P_H2O with increased alkali contents of the suites. This evidence, in conjunction with experimental studies, suggests that increased alkali contents may reflect decreased P_H2O during mantle melting. These features indicate that 2.73 Ga tonalitic rocks are derived from more hydrous mantle sources than 2.68 Ga syenitic rocks, and that the spectrum of late Archean juvenile granitoid rocks is broader than previously recognized. Comparison with Phanerozoic and recent plutonic suites suggests that these Archean suites are subduction related.     

Geochronology and geochemistry of the Early Cretaceous Jigongshan and Qijianfeng batholiths in the Tongbai orogen, central China: implications for lower crustal delamination

The Jigongshan and Qijianfeng batholiths in the Tongbai orogen consist mainly of porphyritic hornblende-biotite monzogranite, biotite monzogranite, and biotite syenogranite, which are variably intruded by lamprophyre, diorite, and syenogranite dykes. Mafic microgranular enclaves commonly occur in the hornblende-biotite monzogranite, whereas surmicaceous enclaves are found in the biotite monzogranite. Both batholiths have zircon U–Pb ages ranging from ca. 139 to 120 Ma, indicating their emplacement in the Early Cretaceous. The hornblende-biotite monzogranite has an adakitic affinity marked by relatively high Sr/Y and (La/Yb) _N ratios, lack of Eu anomalies, low MgO and Ni contents, and Na₂O > K₂O. Its chemical compositions, combined with enriched Sr–Nd isotopic signatures, suggest formation by dehydration melting of mafic rocks in a thickened lower crust. This thickened crust resulted from the Permo-Triassic subduction-collision between the North China and South China blocks and persisted until the Early Cretaceous. The biotite monzogranite and biotite syenogranite have low Al₂O₃, CaO, and Sr contents, low Rb/Sr, FeOt/MgO, and (Na₂O + K₂O)/CaO ratios, and flat HREE patterns with moderate to weak Eu anomalies. They were produced by partial melting of crustal materials under relatively low pressure. Partial melting at different crustal levels could have significantly contributed to mechanical weakening of the crust. The diorite and lamprophyre dykes show linear trends between SiO₂ and major or trace elements on Harker diagrams, with two lamprophyre samples containing normative nepheline and olivine. These rocks have high La/Yb and Dy/Yb ratios, both displaying co-variation with contents of Yb. They were originated from relatively deep lithospheric mantle followed by fractionation of olivine + clinopyroxene + apatite + Fe–Ti oxides. Extensive partial melting in the lithospheric mantle indicates relatively high temperatures at this level. We suggest that the presence of adakitic magmas, thickened but weakened crust and high temperatures in the lithosphere mantle point to lower crustal delamination in the Early Cretaceous in the Tongbai orogen.     

辽南小黑山地区太古宙构造变形序列

采用地质调查和显微镜下观察方法,研究了辽南小黑山区太古宙岩石组成和构造变形特征。小黑山区太古宙岩石包括上壳岩、古老片麻岩和变基性岩脉,它们在小黑山变质岩体中呈包体出现。上壳岩由黑云变粒岩、条带状闪石磁铁石英岩组成;古老片麻岩为条带状角闪黑云斜长片麻岩、条带状角闪斜长片麻岩,原岩为英云闪长岩;变基性岩脉为斜长角闪岩和角闪石岩。上壳岩堆积之后有英云闪长岩侵位,基性脉侵位于上壳岩和英云闪长岩（古老片麻岩）。小黑山区太古宙岩石经历了2幕变形：D₁幕变形主要表现为褶皱构造（DF₁）、与褶皱轴面平行的面理（DS₁）、矿物线理（DL₁）;D₂幕变形在叠加褶皱作用下形成斜歪倾伏褶皱（DF₂）,面理和线理不发育。小黑山区太古宙变质岩中发育的变形序列、构造特征、变形特征、变质条件表明,这2幕构造形迹群属于中部构造相。D1幕变形形成逆冲推覆构造,D₂幕变形形成第Ⅲ型叠加褶皱,它们都是在同方向的水平挤压应力作用下的产物。     

The age and origin of a thick mafic–ultramafic keel from beneath the Sierra Nevada batholith

We present evidence for a thick (∼100 km) sequence of cogenetic rocks which make up the root of the Sierra Nevada batholith of California. The Sierran magmatism produced tonalitic and granodioritic magmas which reside in the Sierra Nevada upper- to mid-crust, as well as deep eclogite facies crust/upper mantle mafic–ultramafic cumulates. Samples of the mafic–ultramafic sequence are preserved as xenoliths in Miocene volcanic rocks which erupted through the central part of the batholith. We have performed Rb-Sr and Sm-Nd mineral geochronologic analyses on seven fresh, cumulate textured, olivine-free mafic–ultramafic xenoliths with large grainsize, one garnet peridotite, and one high pressure metasedimentary rock. The garnet peridotite, which equilibrated at ∼130 km beneath the batholith, yields a Miocene (10 Ma) Nd age, indicating that in this sample, the Nd isotopes were maintained in equilibrium up to the time of entrainment. All other samples equilibrated between ∼35 and 100 km beneath the batholith and yield Sm-Nd mineral ages between 80 and 120 Ma, broadly coincident with the previously established period of most voluminous batholithic magmatism in the Sierra Nevada. The Rb-Sr ages are generally consistent with the Sm-Nd ages, but are more scattered. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd intercepts of the igneous-textured xenoliths are similar to the ratios published for rocks outcroping in the central Sierra Nevada. We interpret the mafic/ultramafic xenoliths to be magmatically related to the upper- and mid-crustal granitoids as cumulates and/or restites. This more complete view of the vertical dimension in a batholith indicates that there is a large mass of mafic–ultramafic rocks at depth which complement the granitic batholiths, as predicted by mass balance calculations and experimental studies. The Sierran magmatism was a large scale process responsible for segregating a column of ∼30 km thick granitoids from at least ∼70 km of mainly olivine free mafic–ultramafic residues/cumulates. These rocks have resided under the batholith as granulite and eclogite facies rocks for at least 70 million years. The presence of this thick mafic–ultramafic keel also calls into question the existence of a “flat” (i.e., shallowly subducted) slab at Central California latitudes during Late Cretaceous–Early Cenozoic, in contrast to the southernmost Sierra Nevada and Mojave regions. Received: 27 December 1997 / Accepted: 11 June 1998     

Hornblende gabbro sill complex at Onion Valley,California, and a mixing origin for the Sierra Nevada batholith

 The steep crest of the Sierra Nevada, California, near Onion Valley, exposes natural cross sections through a mafic intrusive complex that formed as part of the Mesozoic Sierra Nevada batholith. Sheeted sills of hornblende gabbro to hornblende diorite, individually as thick as 1.5 m, form the upper 200 to 300 m of the complex. Thicker, multiply-injected sills, as well as mafic stocks, lie underneath at elevations below 3600 m. Lens-shaped cumulate bodies, as thick as 200 m and more than 700 m broad, lie near the base of the sheeted sill suite. Cumulates are flat-lying, modally layered hornblende gabbro with subsidiary ultramafic olivine hornblendite, plagioclase hornblendite, and late-mobile hornblende-plagioclase pegmatite. Fine grain size, scarce phenocrysts and xenocrysts, and quench mineral textures are evidence that hornblende gabbro sills injected in a largely liquid state and preserve basaltic melt compositions. Most sills reached volatile saturation, as shown by tiny miarolitic cavities that are also widespread in cumulates. Although some sills chilled directly against others, most chilled against septa, millimeters to a few centimeters thick, of medium-grained diorite to granodiorite. Mutually crosscutting relations, as well as chilling, show that the septa were partly molten at the time the sills injected and likely formed the lower portions of an overlying more silicic magma chamber that has since been removed by erosion. Sill compositions range from evolved high-alumina basalt to aluminous andesite with major and trace element abundances similar to those of modern arc magmas. Experimental phase equilibria indicate dissolved water contents near 6 wt% (Sisson and Grove 1993a). The sills show unequivocally that hydrous arc basaltic magmas reached shallow levels in the crust during formation of the largely granodioritic Sierra Nevada batholith. The basaltic magmas appear to have been produced from an enriched mantle source with ⁸⁷Sr/⁸⁶Sr ∼0.7065, ɛNd ∼−4.3, ²⁰⁶Pb/²⁰⁴Pb ∼18.6, ²⁰⁷Pb/²⁰⁴Pb ∼15.6, ²⁰⁸Pb/²⁰⁴Pb ∼38.6. Although crystal fractionation contributed to forming the sill suite and the associated cumulates, nearly constant concentrations of Na₂O, P₂O₅, Nb, Zr, and light rare earth elements in the sills indicate that mixing between sill basaltic and more evolved septa magmas was important for producing sills with andesitic compositions. Average Sierran granodiorite major and trace element concentrations are readily reproduced by a simple mixture of average basaltic sill from Onion Valley and average Sierran low-silica granite. This result supports the inference that Sierran granitoids formed chiefly by mixing between crustal and mantle-derived magmas, although in some cases these crustal melts may have been derived by refusion of earlier mafic intrusions near the base of the crust. The common mafic inclusions (enclaves) in Sierran granodiorites bear a superficial resemblance to Onion Valley mafic sills; however, high concentrations of lithophile elements in the inclusions point to extensive chemical exchange between inclusions and their host magmas. The prevalence of hornblende-rich mafic intrusive rocks at Onion Valley, elsewhere in the Sierra Nevada, and in other shallow subduction batholiths stems from two effects of high melt water concentrations (∼4–6 wt% H₂O). The hydrous parent basaltic and basaltic andesite magmas had low liquidus temperatures, compared to nearly dry basaltic melts, and thus were chilled less during ascent through the crust and were more capable of ascent as liquids. More importantly, their high water concentrations led to low melt densities, higher than granitoid liquids, but comparable to or less dense than partly solidified granitoid magmas. Thus, the hydrous basaltic and basaltic andesite magmas were neutrally or positively buoyant and were capable of penetrating and rising through partly crystallized granitoids and their partly molten source regions to reach upper crustal emplacement levels. Drier basaltic magmas were probably abundant at depth and contributed heat and mass to granite generation, but were insufficiently buoyant to ascend to shallow levels. Received: 2 August 1995 / Accepted: 26 June 1996     

Petrogenesis of the Puerto Edén Igneous and Metamorphic Complex,Magallanes, Chile: Late Jurassic syn-deformational anatexis of metapelites and granitoid magma genesis

A suite of schists, gneisses, migmatites, and biotite granitoids from the Puerto Edén Igneous and Metamorphic Complex (PEIMC) and biotite–hornblende granitoids of the South Patagonian batholith (southern Chile) has been studied. For that purpose, the chemistry of minerals and the bulk rock composition of major and trace elements including Rb–Sr and Sm–Nd isotopes were determined. Mineralogical observations and geothermobarometric calculations indicate high-temperature and low-pressure conditions (ca. 600–700 °C and 3 to 4.5 kbar) for an event of metamorphism and partial melting of metapelites in Late Jurassic times (previously determined by SHRIMP U–Pb zircon ages). Structures in schists, gneisses, migmatites and mylonites indicate non-coaxial deformation flow during and after peak metamorphic and anatectic conditions. Andalusite schists and sillimanite gneisses yield initial ⁸⁷Sr/⁸⁶Sr ratios of up to 0.7134 and εNd₁₅₀ values as low as − 7.6. Contemporaneous biotite granitoids and a coarse-grained orthogneiss have initial ⁸⁷Sr/⁸⁶Sr ratios between 0.7073 and 0.7089, and εNd₁₅₀ values in the range − 7.6 to − 4.4. This indicates that metamorphic rocks do not represent the natural isotopic variation in the migmatite source. Thus, a heterogeneous source with a least radiogenic component was involved in the production of the biotite granitoids. The PEIMC is considered as a segment of an evolving kilometre-sized and deep crustal shear zone in which partial melts were generated and segregated into a large reservoir of magmas forming composite plutons in Late Jurassic times. A biotite–hornblende granodiorite and a muscovite–garnet leucogranite show initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7048 and 0.7061, and εNd₁₀₀ values of − 2.6 and − 1.8, respectively, and are thus probably related to Early Cretaceous magmas not involved in the anatexis of the metasedimentary rocks.     

Hornblendes from Granitic Rocks of the Central Sierra Nevada Batholith, California

Twenty samples of hornblendes from rocks of 14 plutonic unitsin the central Sierra Nevada and Inyo Mountains, California,have been studied in detail. Optical, density, single-crystaland powder X-ray diffraction, and major and minor element chemicaldata are reported. The compositions of the hornblendes show only limited correlationwith the chemistry of the rocks in which they occurred. Hornblendesfrom granitic rocks of the eastern Sierra Nevada and Inyo Mountainshave a wide range of tetrahedral aluminum content which is oftenas low as three-quarters of an atom per formula unit, whereashornblendes from younger granitic rocks elsewhere in the SierraNevada batholith contain more than one atom of tetrahedral aluminumper formula unit. Because an increase of aluminum in tetrahedralco-ordination is considered indicative of higher temperaturesof crystallization, the observed differences in the hornblendessuggest that older plutonic rocks of the batholith may havebeen metamorphosed regionally or may have been affected by widespreadhydrothermal action prior to consolidation of later graniticrocks.     

Geology and field relations of the Wilsons Promontory batholith,Victoria: multiple,shallow-dipping,S-type,granitic sheets

The 1200 km², Early Devonian (395 Ma) Wilsons Promontory batholith is a post-tectonic, high-level, composite body of S-type granites exposed on Wilsons Promontory and its offshore islands. Four plutons and six members are mapped and described. The rocks commonly contain magmatic garnet and cordierite, in addition to biotite, and biotite–quartz pseudomorphs after orthopyroxene. Planar fabrics abound in the batholith, which is characterised by emplacement of shallow-dipping granitic sheets, on a variety of scales. Particle size and density separation occurred during magma flow, and produced a wide variety of structures including layering, pipes and whorls rich in mafic minerals, K-feldspar phenocryst alignments and a notable swarm of enclaves. Local filter pressing may have played a role in the production of accumulations of K-feldspar crystals and the formation of late, tourmaline-bearing leucogranites and quartz veins. Batholith zonation and the distribution of component plutons are inferred to have been formed through sequential intrusion of separate magma batches rather than in situ differentiation. Overall, the batholith appears to consist of saucer-shaped plutons, and it is tilted gently to the east.     

Epidotisation of diorites at Al Hadah,Saudi Arabia: Fluid influx into cooling plutons

Diorite plutons at Al Hadah Saudi Arabia, which constitute part of the pan-African magmatic sequence (ca. 600 Ma), exhibit extensive development of epidote. The epidote alteration is concentrated along veins and dyke margins, and is characterised by transformation of plagioclase (Ab 67)+hornblende+biotite+quartz to epidote+hornblende+tremolite−actinolite+plagioclase (Ab 99)±quartz. The reactions involve addition of CaO and total Fe₂O₃, depletion of MgO, Na₂O and K₂O, with variable gains or losses of SiO₂. Epidotised alteration products are hydrated and oxidised relative to the diorite precursor. The whole rock δ¹⁸O of fresh diorite is + 8.2‰ to + 8.8‰, whereas epidote domains have δ¹⁸O whole rock of +9.8‰ to +10.48‰ and negative Δ¹⁸O_{quartz-plagioclase}, implying oxygen isotope exchange with fluids at low temperatures. Epidotisation is considered to have accompanied influx of fluids into plutons during cooling and contraction. The fluids were probably deep formation waters with relatively high Ca²⁺/Na⁺ ratios, moving up thermal gradient.     

Salinia revisited: a crystalline nappe sequence lying above the Nacimiento fault and dispersed along the San Andreas fault system,central California

Salinia, as originally defined, is a fault-bounded terrane in westcentral California. As defined, Salinia lies between the Nacimiento fault on the west, and the Northern San Andreas fault (NSAF) and the main trace of the dextral SAF system on the east. This allochthonous terrane was translated from the southern part of the Sierra Nevada batholith and adjacent western Mojave Desert region by Neogene-Quaternary displacement along the SAF system. The Salina crystalline basement formed a westward promontory in the SW Cordilleran Cretaceous batholithic belt, relative to the Sierra Nevada batholith to the north and the Peninsular Ranges batholith to the south, making Salinia batholithic rocks susceptible to capture by the Pacific plate when the San Andreas transform system developed. Proper restoration of offsets on all branches of the San Andreas system is a critical factor in understanding the Salinia problem. When cumulative dextral slip of 171 km (106 mi) along the Hosgri–San Simeon–San Gregorio–Pilarcitos fault zone (S–N), or dextral slip of 200 km (124 mi) along the Hosgri–San Simeon–San Gregorio–Pilarcitos–northern San Andreas fault system, is added to the cumulative dextral slip of 315–322 km (196–200 mi) along the main trace of the SAF north of the San Emigdio–Tehachapi mountains, central California, there is a minimum amount of cumulative dextral slip of 486 km (302 mi) or a maximum amount of cumulative dextral slip of 522 km (324 mi) along the entire SAF system north of the Tehachapi Mountains. When these sums are compared with the offset distance (610–675 km or 379–420 mi) between the batholithic rocks associated with the Navarro structural discontinuity (NSD) in northern California, and those in the ‘tail’ of the southern Sierra Nevada granitic rocks in the San Emigdio–Tehachapi mountains, central California, a minimum deficit of from ～100 km (～62 mi) to a maximum deficit of ～189 km (～118 mi) is needed to restore the crystalline rocks associated with the NSD with the crystalline terranes within the San Emigdio and Tehachapi mountains – the enigma of Salinia. Two principal geologic models compete to explain the enigma (i.e. the discrepancy between measured dextral slip along traces of the SAF system and the amount of separation between the Sierra Nevada batholithic rocks near Point Arena in northern California and the Mesozoic and older crystalline rocks in the San Emigdio and Tehachapi mountains in southern California). (i) One model proposes pre-Neogene (>23 Ma), Late Cretaceous or Maastrichtian (<ca. 71 Ma) to early Palaeocene or Danian (ca. 66 Ma) sinistral slip of 500–600 km (311–373 mi) along the Nacimiento fault and of the western flank of Salinia from the eastern flank of the Peninsular Ranges (sinistral slip but in the opposite sense to later Neogene (<23 Ma) dextral slip along and within the SAF system. (ii) A second model proposes that the crystalline rocks of Salinia comprise a series of 100 km- (60 mi-) scale allochthonous (extensional) nappes that rode southwestward above the Rand schist–Sierra de Salinas (SdS) shear zone subduction extrusion channels. The allochthonous nappes are from NW–SE: (i) Farallon Islands–Santa Cruz Mountains–Montara Mountain, and adjacent batholithic fragments that appear to have been derived from the top of the deep-level Sierra Nevada batholith of the western San Emigdio–Tehachapi mountains; (ii) the Logan Quarry–Loma Prieta Peak fragments that appear to have been derived from the top of a buried detachment fault that forms the basement surface beneath the Maricopa sub-basin of the southernmost Great Valley; (iii) The Pastoria plate–Gabilan Range massif that appears to have been derived from the top of the deep-level SE Sierra Nevada batholith; and (iv) the Santa Lucia–SdS massif, which appears to be lower batholithic crust and underlying extruded schist that were breached westwards from the central to western Mojave Desert region. In this model, lower crustal batholithic blocks underwent ductile stretching above the extrusion channel schists, while mid- to upper-crustal level rocks rode southwestwards and westwards along trenchward dipping detachment faults. Salinian basement rocks of the Santa Lucia Range and the Big Sur area record the most complete geologic history of the displaced terrane. The oldest rocks consist of screens of Palaeozoic marine metasedimentary rocks (the Sur Series), including biotite gneiss and schist, quartzite, granulite gneiss, granofels, and marble. The Sur Series was intruded during Cretaceous high-flux batholithic magmatism by granodiorite, diorite, quartz diorite, and at deepest levels, charnockitic tonalite. Local nonconformable remnants of Campanian–Maastrichtian marine strata lie on the deep-level Salinia basement, and record deposition in an extensional setting. These Cretaceous strata are correlated with the middle to upper Campanian Pigeon Point (PiP) Formation south of San Francisco. The Upper Cretaceous strata, belonging to the Great Valley Sequence, include clasts of the basement rocks and felsic volcanic clasts that in Late Cretaceous time were brought to a coastal region by streams and rivers from Mesozoic felsic volcanic rocks in the Mojave Desert. The Rand and SdS schists of southern California were underplated beneath the southern Sierra Nevada batholith and the adjacent Salinia-Mojave region along a shallow segment of the subducting Farallon plate during Late Cretaceous time. The subduction trajectory of these schists concluded with an abrupt extrusion phase. During extrusion, the schists were transported to the SW from deep- to shallow-crustal levels as the low-angle subduction megathrust surface was transformed into a mylonitic low-angle normal fault system (i.e. Rand fault and Salinas shear zone). The upper batholithic plate(s) was(ere) partially coupled to the extrusion flow pattern, which resulted in 100 km-scale westward displacements of the upper plate(s). Structural stacking, temporal and metamorphic facies relations suggest that the Nacimiento (subduction megathrust) fault formed beneath the Rand-SdS extrusion channel. Metamorphic and structural relations in lower plate Franciscan rocks beneath the Nacimiento fault suggest a terminal phase of extrusion as well, during which the overlying Salinia underwent extension and subsidence to marine conditions. Westward extrusion of the subduction-underplated rocks and their upper batholithic plates rendered these Salinia rocks susceptible to subsequent capture by the SAF system. Evidence supporting the conclusion that the Nacimiento fault is principally a megathrust includes: (i) shear planes of the Nacimiento fault zone in the westcentral Coast Ranges locally dip NE at low angles. (ii) Klippen and/or faulted klippen are locally present along the trace of the Nacimiento fault zone from the Big Creek–Vicente Creek region south of Point Sur near Monterey, to east of San Simeon near San Luis Obispo in central California. Allochthonous detachment sheets and windows into their underplated schists comprise a composite Salinia terrane. The nappe complex forming the allochthon of Salinia was translated westward and northwestward ～100 km (～62 mi) above the Nacimiento megathrust or Franciscan subduction megathrust from SE California between ca. 66 and ca. 61 Ma (i.e. latest Cretaceous–earliest Palaeocene time). Much, or all, of the westward breaching of the Salinia batholithic rocks likely occurred above the extrusion channels of the Rand-SdS schists; following this event, the Franciscan Sur-Obispo terrane was thrust beneath the schists, perhaps during the final stages of extrusion in the upper channel. Later, the Sur-Obispo terrane was partially extruded from beneath the Salinia nappe terrane, during which time the upper plate(s) underwent extension and subsidence to marine conditions. Attenuation of the Salinia nappe sequence during the extrusion of the Franciscan Complex thinned the upper crust, making the upper plates susceptible to erosion from the top of the Franciscan Complex near San Simeon, where it is now exposed. In the San Emigdio Mountains, the relatively thin structural thickness of the upper batholithic plates made them susceptible to late Cenozoic flexural folding and disruption by high-angle dip–slip faults. The ～100 km (～62 mi) of westward and northwestward breaching of the Salinia batholithic rocks above the Rand-SdS channels, and the underlying Nacimiento fault followed by ～510 km (～320 mi) of dextral slip from ～23 Ma to Holocene time along the SAF system, allow for the palinspastic restoration of Salinia with the crystalline rocks of the San Emigdio–Tehachapi mountains and the Mojave terrane, resolving the enigma of Salinia.     

朝鲜半岛中南部三叠纪岩浆岩的分布、系列与成因浅析

朝鲜半岛发育大量三叠纪侵入体,主要分布在半岛中部一个北东向走廊地带,该带状区域根据岩性差异,以洪城-高城一线为界分为两个亚带:北亚带(正长岩带/碱性岩带)主要分布在狼林地块南部及京畿地块北部,为碱性系列,以正长岩类为主;南亚带(二长岩带)主要分布在京畿地块南部-岭南地块,多为亚碱性系列,以(纹长)二长岩类为主,部分岩体有闪长岩-辉长岩端元,南部有少量A型花岗岩。这些岩体多形成于ca.230~220Ma,如本文获得的北亚带江北黑云母正长岩228.7±0.8 Ma锆石U-Pb谐和年龄。它们与本区高级变质作用时代接近,分布范围大致对应但稍大,说明它们是同成因的。岩石化学方面,这些岩体均显示高场强元素亏损,轻稀土和大离子亲石元素富集的典型的大陆岩石圈特征。南北亚带相比,北亚带岩体SiO_2和Na_2O等的含量整体上略低,而K_2O、Sr、Ba、La、Eu、Y、Cr等及稀土元素含量略高,轻重稀土分异程度((La/Yb)N)略高。空间上,两个亚带内K_2O/Na_2O和SiO_2有相反的变化趋势:从南向北,两个亚带内K_2O/Na_2O逐渐升高,SiO_2含量逐渐降低;但K_2O+Na_2O和Sr/Y的变化是相似的:南亚带从南向北,北亚带从北向南,两者逐渐降低。我们认为这些岩浆岩为同碰撞或者碰撞后产物,两个亚带之间的界线接近华北和华南古陆缝合带,岩性和成分的空间差异受控于华南古陆(朝鲜半岛南部)向华北古陆(朝鲜半岛北部)之下俯冲所形成的特定壳幔结构。     

Post-collisional melting of crustal sources: constraints from geochronology,petrology and Sr,Nd isotope geochemistry of the Variscan Sichevita and Poniasca granitoid plutons (South Carpathians,Romania)

The Sichevita and Poniasca plutons belong to an alignment of granites cutting across the metamorphic basement of the Getic Nappe in the South Carpathians. The present work provides SHRIMP age data for the zircon population from a Poniasca biotite diorite and geochemical analyses (major and trace elements, Sr–Nd isotopes) of representative rock types from the two intrusions grading from biotite diorite to biotite K-feldspar porphyritic monzogranite. U–Pb zircon data yielded 311 ± 2 Ma for the intrusion of the biotite diorite. Granites are mostly high-K leucogranites, and biotite diorites are magnesian, and calcic to calc-alkaline. Sr, and Nd isotope and trace element data (REE, Th, Ta, Cr, Ba and Rb) permit distinguishing five different groups of rocks corresponding to several magma batches: the Poniasca biotite diorite (P₁) shows a clear crustal character while the Poniasca granite (P₂) is more juvenile. Conversely, Sichevita biotite diorite (S₁), and a granite (S₂*) are more juvenile than the other Sichevita granites (S₂). Geochemical modelling of major elements and REE suggests that fractional crystallization can account for variations within P₁ and S₁ groups. Dehydration melting of a number of protoliths may be the source of these magma batches. The Variscan basement, a subduction accretion wedge, could correspond to such a heterogeneous source. The intrusion of the Sichevita–Poniasca plutons took place in the final stages of the Variscan orogeny, as is the case for a series of European granites around 310 Ma ago, especially in Bulgaria and in Iberia, no Alleghenian granitoids (late Carboniferous—early Permian times) being known in the Getic nappe. The geodynamical environment of Sichevita–Poniasca was typically post-collisional of the Variscan orogenic phase.     

The Pikes Peak batholith,Colorado front range,and a model for the origin of the gabbro—anorthosite—syenite—potassic granite suite

This study of the Pikes Peak batholith includes the mineralogy and petrology of quartz syenite at West Creek and of fayalite-bearing and fayalite-free biotite granite near Mount Rosa; major element chemistry of the batholith; comparisons with similar postorogenic, intracratonic, sodic to potassic intrusives; and genesis of the batholith.The batholith is elongate in plan, 50 by 100 km, composite, and generally subalkalic. It was emplaced at shallow depth 1,040 m. y. ago, sharply transects its walls and may have breached its roof. Biotite granite and biotite—hornblende granite are predominant; quartz syenite, fayalite granite and riebeckite granite are present in minor amounts.Fayalite-bearing and fayalite-free quartz syenite, fayalite-biotite granite and riebeckite granite show a well-defined sodic differentiation trend; the less sodic fayalite-free granites exhibit a broader compositional range and no sharp trends.Crystallization was largely at P_H2O < P_total; P_H2O approached P_total only at late stages. Aplite residual to fayalite-free biotite granite in the north formed at about 1,500 bars, or 5 km depth. Feldspar assemblages indicate late stages of crystallization at about 720°C. In the south ilmenite and manganian fayalite indicate f_O2 of 10^?17 or 10^?18 bars. Biotite and fayalite compositions and the ‘granite minimum’ imply completion of crystallization at about 700°C and 1,500 bars. Nearby fayalite-free biotite granite crystallized at higher water fugacity.All types of syenite and granite contain 5–6% K₂O through a range of SiO₂ of 63–76%. Average Na₂O percentages in quartz syenite are 6.2, fayalite granite 4.2, and fayalite-free granite 3.3 MgO contents are low, 0.03–0.4%; FeO averages 1.9–2.5%. FeO/Fe₂O₃ ratios are high. Fluorine ranges from 0.3 to 0.6%.The Pikes Peak intrusives are similar in mode of emplacement, composition, and probably genesis to rapakivi intrusives of Finland, the Younger Granites of Nigeria, Cape Ann Granite and Beverly Syenite, Mass., and syenite of Kungnat, Greenland, among others — allowing for different levels of erosion. A suite that includes gabbro or basalt, anorthosite, quartz syenite, fayalite granite, riebeckite granite, and biotite and/or hornblende granites is of worldwide occurrence.A model is proposed in which mantle-derived, convecting alkali olivine basaltic magma first reacts with K₂O-poor lower crust of granulite facies to produce magma of quartz syenitic composition. The syenitic liquid in turn reacts with granodioritic to granitic intermediate crust of amphibolite facies to produce the predominant fayalite-free biotite and biotite-hornblende granites of the batholith. This reaction of magma and roof involves both partial melting and the reconstitution and precipitation of refractory phases, as Bowen proposed. Intermediate liquids include MgO-depleted and Na₂O-enriched gabbro, which precipitated anorthosite, and alkali diorite. The heat source is the basaltic magma; the heat required for partial melting of the roof is supplied largely by heats of crystallization of phases that settle out of the liquid — mostly olivine, clinopyroxene and plagioclase.