Petrology of the Chilliwack batholith,North Cascades,Washington: generation of calc-alkaline granitoids by melting of mafic lower crust with variable water fugacity

Calc-alkaline granitoid rocks of the Oligocene-Pliocene Chilliwack batholith, North Cascades, range from quartz diorites to granites (57–78% SiO₂), and are coeval with small gabbroic stocks. Modeling of major element, trace element, and isotopic data for granitoid and mafic rocks suggests that: (1) the granitoids were derived from amphibolitic lower crust having REE (rare-earth-element) and Sr-Nd isotopic characteristics of the exposed gabbros; (2) lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting. The main effect of f_{H 2 O} variation is to change the relative proportions of plagioclase and amphibole in the residuum. The REE data for intermediate granitoids (quartz diorite-granodiorite; Eu/Eu^*=0.84–0.50) are modeled by melting with f_{H 2 O}<1 kbar, leaving a plagioclase + pyroxene residuum. In contrast, data for leucocratic granitoids (leuco-granodiorites and granites; Eu/Eu^* =1.0–0.54) require residual amphibole in the source and are modeled by melting with f_{H 2 O}=2–3 kbar. Consistent with this model, isotopic data for the granitoids show no systematic variation with rock type (⁸⁷Sr/⁸⁶Sr_i =0.7033–0.7043; Nd(0)=+3.3 to +5.5) and overlap significantly with data for the gabbroic rocks (⁸⁷Sr/⁸⁶Sr_i =0.7034–0.7040; Nd(0)=+3.3 to +6.9). The f_{H 2 O} variations during melting may reflect additions of H₂O to the lower crust from crystallizing basaltic magmas having a range of H₂O contents; Chillwack gabbros document the existence of such basalts. One-dimensional conductive heat transfer calculations indicate that underplating of basaltic magmas can provide the heat required for large-scale melting of amphibolitic lower crust, provided that ambient wallrock temperatures exceed 800°C. Based on lithologic and geochemical similarities, this model may be applicable to other Cordilleran batholiths.     

Origin of the Skagit migmatites,North Cascades Range,Washington State

Stromatic and schlieren-type migmatites are a major lithology in the type section of the Skagit Gneiss complex in the North Cascades Range of Washington State, USA. Migmatite mesosomes are chiefly biotite schist, amphibolite, and orthogneiss, in decreasing order of abundance. Leucosomes are predominantly leucotrondhjemites with a very limited range of composition that is nearly independent of associated mesosome type. Melanosomes, consisting mainly of biotite and/or hornblende±garnet, are inconsistently developed and absent in places. The age of migmatization is not well established, but appears to be Late Cretaceous or early Tertiary. This is also the age of syntectonic tonalite to trondhjemite intrusives that are predominant in most parts of the Skagit complex. Although temperatures in excess of 700° C and pressures as high as 10 kb occurred, there is no evidence for widespread partial melting of the mesosomes with which the migmatites are closely associated. Mass balance calculations preclude an origin by injection of a silicate melt or hydrothermal fluid unless accompanied by metasomatic replacement reactions. Mass balance relationships also show that the Skagit migmatites could not have formed solely by closed system processes such as partial melting or metamorphic segregation, unless the mesosomes present were not the protolith from which the migmatites formed. Field, petrographic and geochemical data indicate that an origin by migmatization of a missing mesosome is quite unlikely. The most feasible process of migmatization appears to be infiltration of an aqueous fluid into a metamorphic protolith along fracture or foliation planes. This triggers a variable degree of metamorphic segregation or possibly minor partial melting. Unmixing of leucosomes and melanosomes from the mesosome protolith must be accompanied by metasomatic replacement, but the total mass transfer required is only a few wt%.     

The North Fork porphyry copper deposit of the Washington Cascades

The North Fork porphyry copper deposit in the Cascade volcanic arc of Washington displays copper mineralization confined to the potassic (biotite) and phyllic (sericite) alteration zones. No secondary potassium feldspars have been found in either alteration zone; moreover, chemical analyses indicate the potassic alteration zone contains a low K₂O content. Argillic and propylitic zones are also recognized, but these are barren of hypogene copper mineralization. Biotite-chalcopyrite intergrowths in the deposit have been given a 9.9 K-Ar age. Thus it is not only one of the youngest deposits dated in the western Cordillera, but it is associated with a volcanic arc which has no current Benioff zone seismic array or accompanying trench. The deposit appears to have developed during the period when coupling of the North American and Juan de Fuca plates probably inhibited subduction under the Cascade volcanic arc.     

Symplectite breakdown of Ca-rich almandines in upper amphibolite-facies Skagit Gneiss,North Cascades,Washington

In upper amphibolite-facies Skagit Gneiss, certain rocks, usually carrying hornblende, display post-kinematic breakdown of almandine-rich garnets to symplectite consisting of plagioclase plus biotite and/or hornblende. Other almandine-rich garnets, widespread in hornblende-free schists and gneisses, remained stable to the end of the metamorphic cycle. Analysed garnets with symplectite coronas have 14 to 30 mol.% grandite. A set of garnets free of such coronas have 6 to 9% grandite and contain relatively more pyrope.In all symplectite-bearing rocks selected for mineral analyses, symplectite plagioclase is more calcic than the earlier main-fabric plagioclase. Fe and Mn are higher, and Mg and Ti lower, in symplectite than in main-fabric biotite and hornblende, except where late re-equilibration has been locally accomplished. Main-fabric biotite, hornblende, and cummingtonite partly preserve a record of original element partitioning between these phases and garnet, indicating that equilibrium was approached during the essentially syn-kinematic main stage of the metamorphism. Between the main-stage phases and their post-kinematic symplectite counterparts, equilibrium has rarely been attained, despite high T and presence of H₂O. Instead, there are sequences of arrested chemical exchanges and of highly incomplete attainments of successive equilibria. The principles revealed by this study are thought to be more broadly applicable to questions of equilibrium vs. disequilibrium in regional-metamorphic sequences.Combined with petrographic-petrologic data, the mineral analyses permit one to calculate model equations that quantitatively describe symplectite-forming reactions, including the amounts of materials added to and removed from garnets transformed into symplectites. Na, K, minor Ba, H₂O, and part of the Ti present were added; major portions of the garnets' Fe and of their subordinate Mn were removed. Transfers of Mg, Al, and Si during symplectite formation were minor, if any. Source and disposal of added and removed substances are discussed. The mass balance of symplectite formation ranges from significant losses in all hornblende-bearing reactions examined to minor gains in some only-biotite-bearing reactions. V is positive for all reactions examined and ranges from a few per cent for subordinate only-hornblende-bearing symplectites to somewhat over 50% for certain only-biotite-bearing symplectites; intermediate values are obtained for symplectites carrying both hornblende and biotite.     

Miscibility of Tremolite and Hornblende in Progressive Skagit Metamorphic Suite, North Cascades, Washington

Tremolite-hornblende relationships are reported for high gradeand relatively low grade zones within the Barrovian type SkagitSuite, using rocks that range widely enough in composition toqualify as potential hosts of any calcic amphibole from practicallyA1-free to A1-rich types. Out of over 100 samples analysed bymicroprobe, 22 representative analyses are listed, with end-membercalculations. In the sillimanite grade core of the Skagit Suite, calcic amphibolesfrom various, commonly metasomatized metamorphic ultramaficsand genetically related hornblendites, from amphibolites, schistsand gneisses, and from some metamorphosed impure dolomites showcontinuous solid solution between tremolite and highly aluminoushornblendes ranging from almost Fe-free to moderately Fe-richtypes. A1_total/A1^IV is nearly constant and approximates 1?44.The second group of calcic amphiboles studied is from the lower-mediumrange of the epidote amphibolite facies (comprehensively defined),that is, from near and above the oligoclase isograd. Host rocksare variously metasomatized meta-peridotites, and amphibolitesand schists. There is a large compositional gap between analysesof tremolites and of moderately to highly aluminous hornblendes.A1_total/A1^IV approximates 1?73 both in the tremolite and hornblendefields, as against 1?44 at high grade. Fe-poor hornblendes,such as are stable at high grade, were not found at the lowergrade. At both grades, Al shows good overall correlation withNa+K, with A occupancy, and with Ti (with more Ti at high grade). The compositional gap between tremolite and hornblende analysesfrom the lower grade rocks does not necessarily define a solvusbecause no tremolite-hornblende pairs were found. Rather, theanalyses provide outer limits on the possible width of a solvusat this grade (T). However, certain data suggest that a truemiscibility gap not only exists but probably is not a greatdeal narrower than the gap between the actual analyses. Besides,the proposed restriction on A1/Fe ratios at this grade wouldreduce the probability of finding tremolite-hornblende pairs.     

湖南东坡矿田花岗岩类岩石学基本特征

东坡矿田所处湘南区域,岩浆岩比较发育,由老到新依次为花岗岩、花岗斑岩脉、辉绿玢岩脉,主要分燕山早期和晚期两期五次侵入就位。在统计前人所做的K-Ar、Rb-Sr同位素测定的基础上,笔者对各期依次所形成的主要矿体特征进行总结概述。     

Petrology of plagiogranites of the Yenisei Batholith, Western Sayan

The tonalite-plagiogranite (tonalite-trondhjemite) association only occasionally occurs in the form of large granitoid bodies, such as the Yenisei Batholith (>500 km² in area). The granitoids of the Yenisei Batholith belong to Na-rich tholeiitic rock series and differ from granitoids of the calc-alkaline series in having lower contents of alkalis and alumina (12–14 wt % Al₂O₃) and low contents of granitophile elements (Rb, Li, Cs, Be, Nb, Ta, and W), Cr, and Ni. The Cr/V (<0.10) and Rb/Sr (0.01–0.1) ratios of these rocks are at a minimum, and their K/Rb (600–1000) and Na/K (5–10) ratios are at a maximum compared to those of the rocks of the most widely spread granitoid batholiths. The plagiogranites typically have REE concentrations higher than those in oceanic plagiogranites and display weakly fractionated REE patterns (La/Yb = 1.4–3.4) with weak (or without) Eu anomalies. The lower initial Sr ratios of these rocks (0.704) and their relatively high concentrations of Pb, Zr, and B testify to the predominantly mantle provenance of their protolithic material. Geological and geochemical characteristics of the Yenisei pluton suggest that its genesis can be considered within the scope of the model of retrograde-type magmatic replacement and that the batholith was produces by the earliest granitization processes in the oceanic crust. The granitic melt was derived at low pressures (<5 kbar) and intermediate temperatures (～700°C), at the inflow of an aqueous transmagmatic fluid into the magma-generating area and the subsequent fluid-magmatic differentiation. Considering the volumes and compositions of rocks composing the Yenisei Batholith, the latter can be attributed, similarly to other typical granitoid batholiths, to crustal plutons, which differ from both oceanic plagiogranites in ophiolitic belts and continental trondhjemites. The rocks can be regarded as an individual geochemical type of crustal plagiogranites.     

Fluid flow,metasomatism and amphibole deformation in an imbricated ophiolite,North Cascades,Washington

Metasomatic tremolite-rich mylonites are widespread in imbricate thrust slices of ultramafic rocks of the ophiolitic Ingalls Complex in Washington State. Protoliths for these amphibolite-facies mylonites were peridotite and serpentinite. Abundant syntectonic tremolite veins in the ultramafites record narrowly channelized flow of infiltrating fluids, whereas metasomatic mylonite zones record more pervasive flow. Fluids were probably released mainly by prograde devolatization reactions within serpentinite and mafic ophiolitic rocks that experienced earlier hydrothermal metamorphism.Olivine apparently deformed by dislocation creep in the mylonites. In the tremolite-rich rocks, locally preserved amphibole porphyroclasts deformed mainly by microfracturing. Acicular tremolites, which dominate the mylonites, form syntectonic overgrowths on porphyroclasts and probably record diffusive mass transfer which may have accompanied cataclasis. Acicular tremolites subsequently were folded and define both post-crystalline crenulations and polygonal arcs.Fluid flow, deformation and metamorphism were apparently complexly interrelated in the imbricate zone. Thrusts juxtaposed contrasting rock types that were sources and sinks for fluids, and shear zones focused fluid flow. Metamorphism probably facilitated deformation through the release of fluids during dehydration reactions. High fluid pressure may have led to hydraulic fracturing and may have controlled strain softening in the tremolitic mylonite zones as it favored microcracking and diffusive mass transfer over dislocation creep. Infiltrating metasomatic fluids probably play an important role in the evolution of shear zones in many ultramafic bodies during medium-grade metamorphism.     

The Petrology and Geochemistry of Calc-Alkaline Andesites on Shodo-Shima Island, SW Japan

Petrographical and geochemical characteristics of calc-alkalineandesites on Shodo-Shima Island, SW Japan, having bulk compositionslargely identical to the continental crust, are presented. Thefollowing petrographic observations suggest a role for magmamixing in producing such andesite magmas: (1) two types of olivinephenocrysts and spinel inclusions, one with compositions identicalto those in high-Mg andesites and the other identical to thosein basalts, are recognized in terms of Ni–Mg and Cr–Al–Fe³⁺relations, respectively; (2) the presence of orthopyroxene phenocrystswith mg-number >90 suggests the contribution of an orthopyroxene-bearinghigh-Mg andesite magma to production of calc-alkaline andesites;(3) reversely zoned pyroxene phenocrysts may not be in equilibriumwith Mg-rich olivine, suggesting the involvement of a differentiatedandesite magma as an endmember component; (4) the presence ofvery Fe-rich orthopyroxene phenocrysts indicates the associationof an orthopyroxene-bearing rhyolitic magma. Contributions fromthe above at least five endmember magmas to the calc-alkalineandesite genesis can also provide a reasonable explanation ofthe Pb–Sr–Nd isotope compositions of such andesites. KEY WORDS: calc-alkaline andesites; high-Mg andesites; magma mixing; continental crust; SW Japan     

Petrogenesis of Mafic Inclusions in Granitoids of the Adamello Massif, Italy

ABSTRACT The Tertiary Adamello calc-alkaline batholith in the ItalianAlps is characterized by tonalite and granodiorite plutons associatedwith small mafic/ultramafic intrusions, syn-plutonic mafic dykesand sills, and ubiquitous mafic inclusions. In the southernmostVal Fredda Complex, syn-plutonic hornblende-gabbro and dioritesheets pass laterally into swarms of mafic inclusions intermingledwith tonalite. Petrological and geochemical data show that themafic sheets represent hydrous mafic magmas derived by fractionalcrystallization from parental hydrous basalt and picro-basalt.The fractionation process is recorded by inclusions of spinel,olivine, and pyroxenes in the cores of hornblende phenocrystsand by the widespread occurrence of calcic plagioclase. Fractionationoccurred at high pressure (Ptoul = 8–10 kb) before intrusionat shallow depths (P_total 2 kb). Geothermometry and meltingexperiments at PH₂O= 1 kb, combined with textural evidence,indicate that the mafic sheets were emplaced at temperaturesof 1050–1100C into hot, but consolidated, granitoid hostrocks. Transfer of heat and hydrous fluids from the sheets remobilizedthe host rocks into crystal-mush, which in turn disrupted thesheet margins to form mafic inclusions. Dynamic crystallizationexperiments indicate that the mafic inclusions and sheet marginswere quenched to temperatures below 970 C, resulting in thefailure of the high-temperature liquidus phases olivine andclinopyroxene to nucleate and the formation of acicular hornblendeand plagioclase. Several other Adamello plutons display syn-plutonicintrusions and mafic inclusions with comparable features tothe Val Fredda Complex. The Adamello mafic inclusions show pronounced enrichments incertain trace elements compared with values expected by fractionalcrystallization and magma mixing. K, Rb, Ba, Y, heavy REE, Mn,and Nb have absolute abundances in the inclusions greater thanthe interiors of neighbouring mafic sheets and, in some cases,than the host granitoids. Many inclusions also display leucocratichaloes, margins rich in ferromagnesian minerals and abundantgroundmass biotite. These features are interpreted in termsof a three-stage evolution. (1) A blob of mafic magma is quenchedby the felsic host to form a rigid crystal-rich inclusion containingan interstitial melt phase. Leucocratic haloes and crenulatemargins to the inclusions form as a result of volume contractionon cooling. (2) The more mobile elements (notably the alkalisand H₂O) diffuse between the melt phases of host and inclusion.Using published experimental data on the variation of melt fractionwith temperature in hydrous basic and acid magmas, it is arguedthat the observed diffusion of K from host to inclusion requiresinteraction temperatures of >900C. Reaction of K-enrichedmelt with existing hornblende in the inclusion forms biotite,which sequesters and concentrates further K₂O and other alkalineelements. (3) During protracted cooling the mafic inclusionsequilibrate with interstitial melt in the host granitoid. Equilibriumpartitioning of heavy REE and Y into the mafic minerals in theinclusion results in the observed enrichments. Magnetite likewiseconcentrates Nb and Mn. It is proposed that mafic inclusions form in the waning stageof pluton evolution when the granitoid magma is sufficientlyconsolidated to allow the penetration of mafic intrusions, butsufficiently hot to be readily remobilized and disrupt theseintrusions to form mafic inclusions. Subsequent chemical equilibrationof mafic inclusions with their host can have a marked impacton the trace element chemistry of both rock types. Granitoidswhich have experienced extensive interaction with mafic inclusion-formingmagmas may undergo significant depletion in those trace elementswhich partition strongly into the minerals of the mafic inclusion.     

Role of slope stability in cumulative impact assessment of hydropower development: North Cascades, Washington

Two environmental assessments considered the potential cumulative environmental impacts resulting from the development of eight proposed hydropower projects in the Nooksack River Basin and 11 proposed projects in the Skagit River Basin, North Cascades, Washington, respectively. While not identified as a target resource, slope stability and the alteration of sediment supply to creeks and river mainstems significantly affect other resources. The slope stability assessment emphasized the potential for cumulative impacts under disturbed conditions (e.g., road construction and timber harvesting) and a landslide-induced pipeline rupture scenario. In the case of smallscale slides, the sluicing action of ruptured pipeline water on the fresh landslide scarp was found to be capable of eroding significantly more material than the original landslide. For large-scale landslides, sluiced material was found to be a small increment of the original landslide. These results predicted that hypothetical accidental pipeline rupture by small-scale landslides may result in potential cumulative impacts for 12 of the 19 projects with pending license applications in both river basins.     

Debris flow magnitude,frequency, and precipitation threshold in the eastern North Cascades,Washington, USA

We examine the magnitude, frequency, and precipitation threshold of the extreme flood hazard on 37 low-order streams in the lower Stehekin River Valley on the arid eastern slope of the North Cascades. Key morphometric variables identify the magnitude of the hazard by differentiating debris flood from debris flow systems. Thirty-two debris flow systems are fed by basins?<?6 km² and deposited debris cones with slopes?>?10°. Five debris flood systems have larger drainage areas and debris fans with slopes 7–10°. The debris flood systems have Melton ruggedness ratios from 0.42–0.64 compared to 0.78–3.80 for debris flow basins. We record stratigraphy at seven sites where soil surfaces buried by successive debris flows limit the age of events spanning 6000 years. Eighteen radiocarbon ages from the soils are the basis for estimates of a 200 to1500-year range in recurrence interval for larger debris flows and a 450?±?50-year average. Smaller events occur approximately every 100 years. Fifteen debris flows occurred in nine drainage systems in the last 15 years, including multiple flows on three streams. Summer storms in 2010 and 2013 with peak rainfall intensities of 7–9 mm/h sustained for 8–11 h triggered all but one flow; the fall 2015 event on Canyon Creek occurred after 170 mm of rain in 78 h. A direct link between fires and debris flows is unclear because several recent debris flows occurred in basins that did not burn or burned at low intensity, and basins that burned at high intensity did not carry debris flows. All but one of the recent flows and fires occurred on the valley’s southwest-facing wall. We conclude that fires and debris flows are linked by aspect at the landscape scale, where the sunny valley wall has flashy runoff due to sparse vegetation from frequent fires.

     

Oxygen isotope geothermometry and stability of lawsonite and pumpellyite in the Shuksan Suite,North Cascades,Washington

Metamorphic temperatures of 330°–400° C are inferred for rocks from the Shuksan blueschist terrane in the North Cascades, Washington. The temperatures are calculated from ¹⁸O fractionations between coexisting quartz and magnetite using the equations of Bottinga and Javoy (1973). Pressures of approximately 7 kilobars are indicated by the Jadeite content of clinopyroxene coexisting with quartz+albite. Published experimental and theoretical studies of the stability of lawsonite and pumpellyite are consistent with the oxygen isotope temperatures and occurrence of these minerals in the Shuksan Suite.     

Petrology of the Albany and Torbay Adamellite Plutons,near Albany,Western Australia

The Albany and Torbay Adamellites are composite plutons emplaced in Pre‐cambrian gneisses of the Albany‐Esperance Block in the vicinity of Albany, Western Australia. The gneissic country rocks have been metamorphosed to the lower granu‐lite facies at Albany and the upper amphibolite facies at Torbay. Granitized aureoles about 1 km wide, metasomatically enriched in SiO₂, K₂O, and various trace elements commonly including Rb, Ba, La, Pb, and Th, are developed in the gneisses around both plutons. Field relations suggest late‐kinematic magmatic emplacement of the Adamellites in the catazone. Both show chemical variation trends comparable with the trends normally associated with fractional crystallization of calc‐alkali magmas, and their normative compositions correspond with the thermal trough in the system An‐Ab‐Or‐Q‐H₂O at 4–7 kb P_H2o, suggesting an origin involving crystal‐liquid equilibria at a water vapour pressure of about this value. The initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7118 for the Albany Adamellite is consistent with derivation of the magma from crustal rocks. The late‐kinematic field characteristics of the plutons and the limited isotopic data available are compatible with emplacement and crystallization during the closing stages of orogeny and regional metamorphism. The magmas are believed to have been generated not at their present sites of emplacement, but in a deeper, higher‐temperature zone of the crust, with magma generated during an earlier phase of the orogeny.     

大别岳西地区花岗岩类岩石学及其成因

岳西地区花岗岩类主要由主簿原和白马尖二长花岗岩－正长花岗岩岩基、花岗闪长岩－石英二长闪长岩小岩体和晚期的淡色碱长花岗岩岩体组成。主要岩体形成于燕山晚期,为造山后花岗岩。岩基中花岗岩的暗色矿物为黑云母,主簿原花岗岩含有褐帘石,岩石化学显示为过铝质（Ａ／ＣＮＫ约为１．１）,稀土配分为右倾的、中等负铕异常（Ｓｍ／Ｅｕ为０．１６～０．２５）曲线。花岗闪长质的、没有变形的小岩体中普遍含有角闪石和榍石,岩石化学表现为准铝质（Ａ／ＣＮＫ为０．８～０．９,Ａ／ＮＫ为１．５～１．８）,稀土配分为右倾的无铕异常（或略有正铕异常）的曲线,但其形成时代和εＮｄ（０）值与主要岩基相同。花岗岩类的εＮｄ（０）值均为较大的负值（－１７～－２６）,在εＮｄ（０）－εＳｒ（０）图上,表现为一条水平带状分布,显示其源岩为存留时间很长的古老地壳。     

Stable isotope compositions of olivine and dolomite in peridotites formed by deserpentinization,Darrington area,North Cascades,Washington

Oxygen isotope analyses of five olivines from the Darrington peridotite, Washington, yield δO¹⁸ values of +7.3 to +8.9%. which are consistent with derivation of these rocks from a serpentinite precursor. The isotopic data are compatible with mineralogical, textural and chemical evidence that most of the Darrington peridotites have formed by deserpentinization. Olivine from a single, petro-graphically distinct peridotite sample has a δO¹⁸-value of +5.2%. which is within the field of high-temperature olivines. The isotopic and textural evidence indicate that this is a partially recrystallized peridotite tectonite.Oxygen and carbon isotope analyses of dolomites from olivine-carbonate rocks indicate that they could have originated by introduction of atmospheric CO₂ via meteoric waters during the formation of ophidolomites or ophicalcites. Subsequent metamorphism and reequilibration have modified the δO¹⁸-values.     

新疆托里别鲁阿嗄希地区花岗岩类的岩石学和成因

别鲁阿嘎希花岗闪长岩－石英闪长岩带与金矿化有十分密切的关系。锆石Ｕ－Ｐｂ法测得别鲁阿嘎希石英闪长岩年龄为３６９Ｍａ。斜长石环带成分振荡,与黑云母共生的角闪石成分显示为富镁。岩石地球化学特征表明,这些岩石为幔源的拉斑玄武岩岩浆和地壳部分熔融形成岩浆混合后的产物。     

“北祁连蛇绿岩及其构造背景野外研讨会”在甘肃举行

别鲁阿嘎希花岗闪长岩－石英闪长岩带与金矿化有十分密切的关系。锆石Ｕ－Ｐｂ法测得别鲁阿嘎希石英闪长岩年龄为３６９Ｍａ。斜长石环带成分振荡,与黑云母共生的角闪石成分显示为富镁。岩石地球化学特征表明,这些岩石为幔源的拉斑玄武岩岩浆和地壳部分熔融形成岩浆混合后的产物。     

Origin of Mafic Enclaves in the Dinkey Creek Pluton, Central Sierra Nevada Batholith, California

Two types of mafic enclaves occur in the Dinkey Creek pluton:ubiquitous microgranular enclaves, and rare gabbroic enclaves.Common petrographic features of the microgranular enclaves are:(1) fine grain-size, (2) abundant acicular apatite, and (3)plagioclase zoned from bytownitic cores to andesine-labradoriterims, with sharp boundaries between these main zones. Subordinateoscillatory variations are commonly superimposed on both coresand rims. It has been found by secondary ion mass spectrometrythat the rims are identical in major and trace element compositionto plagioclase in the tonalite, which suggests crystallizationfrom the same or similar magmas. The gabbroic enclaves are composedpredominantly of hornblende (50–85%) and appear to bemagmatic segregations. The microgranular enclaves and host rocks display two convergingtrends on silica variation diagrams for Fe₂O₃, TiO₂, Al₂O₃,Zn, and Zr. The dominant trend is defined by small microgranularenclaves, by samples from a large (20 m?30 m) microgranularenclave, and by the Dinkey Creek tonalites and granodiorites.The subordinate trend covers tholeiltic dikes and tonalitich and converges with the Dinkey Creek host rocks at 61 wt.%SiO₂ Alkali and alkaline earth elements exhibit greater variabilitythan the above constituents and appear to be either enrichedor depleted as required for equilibrium with the host rocks.Low CaO and Sr concentrations in small enclaves (<30 cm)apparently reflect a lower modal abundance of calcic plagioclaseand more sericitization of this feldspar as compared with theplagioclase of the large microgranular enclave. The large enclaveis also richer in MgO than the small enclaves. With the exceptionof the alkali elements, the major element compositions of themicrogranular enclaves approach high-Al basaltic to andesiticcom positions. In one analyzed microgranular enclave, low La/Cerelative to chondrites and more abundant HREE than in othermicrogranular samples suggest that it may also contain minorcumulus hornblende. The petrographic and whole-rock geochemical relations, and theplagioclase compositions in the microgranular enclaves and theirhost rocks, indicate that the microgranular enclaves representmixtures of quenched basalts and Dinkey Creek tonalites. Itappears that dikes of high-alumina basalt were intruded intothe lower, tonalitic portions of the Dinkey Creek pluton, wherethey were partially quenched along an interface with overlyingtonalitic magma. Large portions of residual liquid in the partiallyquenched basalts permitted mixing with the overlying magma toform a hybrid zone. This zone was then disaggregated, yieldingthe enclaves, and they were dispersed throughout the upper partof the Dinkey Creek magma chamber. Subsequent crystallizationof tonalitic melt within the enclaves produced the zoned plagioclaseand re-equilibrated hornblende and biotite in the enclaves tothe Dinkey Creek magmatic conditions. Scouring disrupted hornblende-richmagmatic segregations and produced the gabbroic enclaves.     

川西双江口地区花岗岩的岩石学、地球化学特征及其成因研究

双江口地区出露的花岗岩主要为二云二长花岗岩和似斑状黑云花岗岩,二者呈侵入接触关系,在岩体特征、岩石颜色、结构、矿物组成及后期改造等方面存在较为明显的差异。岩相学特征表明二云二长花岗岩形成晚于似斑状花岗岩,岩石地球化学特征表明二者成因为部分熔融,物源为壳源,形成环境为同碰撞-造山带环境,形成时间为印支晚期。