西藏冈底斯南缘雄村铜金矿床成矿斑岩厘定及其锆石U-Pb和黑云母Ar-Ar年龄分析

雄村特大型斑岩铜金矿床主要以细脉浸染状产于强烈蚀变岩石中,赋矿岩石原岩成因类型存在争议。本文对多个赋矿蚀变岩石作了系统光薄片显微鉴定,在多个蚀变较弱的矿化样品中发现赋矿岩石具斑状结构,其基质主要为钾长石,斑晶主要为斜长石、钾长石及少量石英,显示石英正长斑岩及二长斑岩(少量)矿物组成特征。结合前人工作,可以认为雄村铜金矿床赋矿岩石为正长斑岩、火山岩及少量二长斑岩。正长斑岩发育斑岩铜金矿床成矿早期常见的钾硅化蚀变及磁铁矿化蚀变,锆石具高的Ce4+/Ce3+比值(334~3084,平均值为1169),显示高氧逸度岩浆特征,和世界斑岩铜金矿床成矿岩体一致;这表明石英正长斑岩为雄村铜金矿床成矿岩体。石英正长斑岩锆石LA-ICP-MS U-Pb年龄为173.7±2.1Ma(MSWD=0.23),石英正长斑岩钾化阶段形成的黑云母40Ar/39Ar坪年龄为48.3±0.9Ma(MSWD=1.58),远小于锆石U-Pb年龄却与矿区东北部始新世花岗岩基的年龄一致,显示Ar-Ar年龄受后期地质事件影响而发生重置。通过上述研究,可以认为雄村铜金矿床为与石英正长斑岩有关的斑岩型矿床,形成时代约173Ma,和新特提斯洋洋壳向北俯冲诱发的岩浆事件有关,矿区内云母受后期地质事件影响重置,不能记录其形成时代。     

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.     

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.     

Two “S-type” granite suites with low initial 87Sr/86Sr ratios from the New England Batholith,Australia

Two “S-type” (pelitic) granite suites from the New England Batholith, N.S.W., have Upper Carboniferous ages, indicating that they predate by 40 m.y. the intrusion of hornblende biotite granites, and are the oldest plutons of the batholith. Mineralogically and geochemically both suites have “pelitic” characteristics, one suite containing an Al-rich biotite, muscovite and cordierite, the other an Al-rich biotite and rare pyrope-almandine garnet. Low initial ⁸⁷Sr/⁸⁶Sr ratios of 0.706 for both suites probably reflect the volcanoclastic nature and young age of the sedimentary source of these granites at the time of melting. The age of the suites coincides with the last stages of (Andean type?) volcanism along an andesite/dacite volcanic chain to the west, suggesting an origin for the “S-type” granitic magma by partial melting of deformed sediments marginal to a continental region.     

新疆阿尔泰巴斯铁列克钨多金属矿区花岗岩黑云母特征及成岩成矿意义

黑云母是花岗质岩石中常见的造岩矿物,其成分可以有效指示花岗岩形成的物理化学条件和岩石成因。巴斯铁列克矿床是近年来在新疆阿尔泰造山带南缘发现的首例二叠纪矽卡岩型钨多金属矿床。矿区出露多种类型二叠纪含钨花岗岩。为理清花岗质岩体之间、岩体与钨多金属矿化之间的关系,文章采用电子探针测定了黑云母花岗岩、二长花岗岩、二云母花岗岩和钾长花岗岩中的黑云母成分。结果表明,所有黑云母具有富铁、高铝、贫镁特征,含铁指数(Fe²⁺/(Mg+Fe²⁺))为0.66~0.80,二云母花岗岩属铁质黑云母而黑云母花岗岩、二长花岗岩和钾长花岗岩属铁叶黑云母。所有岩石是具有A型特征的I型花岗岩。不同类型岩石中黑云母的成分差异与岩浆来源、分异演化程度有关。二云母花岗岩中黑云母的w(MgO)与结晶温度最高,与黑云母平衡流体的log(fHF/fHCl)值（-1.13~-1.25）最低,log(fH₂O/fHF)值（4.64~4.96）最高,母岩浆相对富Cl;黑云母花岗岩中log(fHF/fHCl)值最高,log(fH₂O/fHF)最低,与二长花岗岩是同一岩浆房不同演化阶段的产物,与二云母花岗岩和钾长花岗岩属不同的岩浆体系,母岩浆相对富F元素。黑云母花岗岩与W矿化关系更密切。     

Distribution of elements in biotite-hornblende pairs and in an orthopyroxene-clinopyroxene pair from zoned plutons,northern Sierra Nevada,California

Distribution of major and minor elements has been determined for five hornblende-biotite pairs from hornblende-biotite quartz diorite and monzotonalite and for a clinopyroxene-orthopyroxene pair from pyroxene diorite collected from the border zones and centers of zoned plutons in the northern Sierra Nevada, California. The distribution coefficients K _d [Mg/Fe] for biotite/hornblende are of the same magnitude (0.61–0.67) for both the mafic border zone and the silicic center.For comparison, K _D [Mg/Fe] values for biotite/hornblende from plutonic rocks of the central Sierra Nevada and the southern California batholith were calculated from data published by others. Rocks of the oldest age group (ca. 150 m.y.) in the central Sierra Nevada have an average distribution coefficient, K _D , of 0.64, close to the average K _D in the study area, where K-Ar dates are 143 to 129 m.y. The intermediate age group has an average K _D =0.81, and the youngest group has K _D =0.77. K _D [Mg/Fe] for biotite/hornblende from the southern California batholith is 0.83, close to the average of the intermediate age group in the central Sierra Nevada. The calculated difference in pressure of crystallization between rocks of the Feather River area and the southern California batholith is 1 kb; the rocks of the Feather River area being crystallized at a higher pressure. This is in good agreement with the low-pressure contact metamorphism in the south (pyroxene hornfels facies), as compared with a medium-pressure metamorphism around the northern plutons, where andalusitesillimanite-cordierite and andalusite-staurolite subfacies of the amphibolite facies indicate pressures of about 4 kb.Trace elements Cr, V, Ni, Co, Ga are distributed equally between biotite and hornblende, whereas Ba and possibly Cu are concentrated in biotite and Sr and Sc and possibly Zr in hornblende.Publication authorized by the Director, U.S. Geological Survey.     

Petrogenesis of mafic microgranular enclaves (MMEs) in the oligocene-miocene granitoid plutons from northwest Anatolia,Turkey

Mafic microgranular enclaves (MMEs) in host granitoids can provide important constraints on the deep magmatic processes. The Oligocene-Miocene granitoid plutons of the NW Anatolia contain abundant MMEs. This paper presents new hornblende Ar-Ar ages and whole-rock chemical and Sr-Nd isotope data of the MMEs from these granitic rocks. Petrographically, the MMEs are finer-grained than their host granites and contain the same minerals as their host rocks (amphibole + plagioclase + biotite + quartz + K-feldspar), but in different proportions. The Ar-Ar ages of the MMEs range from 27.9 ± 0.09 Ma to 19.3 ± 0.01 Ma and are within error of their respective host granitoids. The MMEs are metaluminous and calc-alkaline, similar to I-type granites. The Sr-Nd isotopes of MMEs are 0.7057 to 0.7101 for ⁸⁷Sr/⁸⁶Sr and 0.5123 to 0.5125 for ¹⁴³Nd/¹⁴⁴Nd, and are similar to their respective host granitoids. These lithological, petrochemical and isotopic characteristics suggest that the MMEs in this present study represent chilled early formed cogenetic hydrous magmas produced during a period of post-collisional lithospheric extension in NW Anatolia. The parental magma for MMEs and host granitoids might be derived from partial melting of underplated mafic materials in a normally thickened lower crust in a post-collisional extensional environment beneath the NW Anatolia. Delamination or convective removal of lithospheric mantle generated asthenospheric upwelling, providing heat and magma to induce hydrous re-melting of underplated mafic materials in the lower crust.     

Origin of the Newly Discovered Zhunuo Porphyry Cu-Mo-Au Deposit in the Western Part of the Gangdese Porphyry Copper Belt in the Southern Tibetan Plateau, SW China

The newly discovered Zhunuo porphyry Cu-Mo-Au deposit is located in the western part of the Gangdese porphyry copper belt in southern Tibet, SW China. The granitoid plutons in the Zhunuo region are composed of quartz diorite porphyry, diorite porphyry, granodiorite porphyry, biotite monzogranite and quartz porphyry. The quartz diorite porphyry yielded zircon U-Pb ages of 51.9±0.7 Ma(Eocene) using LA-ICP-MS, whereas the diorite porphyry, granodiorite porphyry, biotite monzogranite and quartz porphyry yielded ages ranging from 16.2±0.2 to 14.0±0.2 Ma(Miocene). CuMo-Au mineralization is mainly hosted in the Miocene granodiorite porphyry. Samples from all granitoid plutons have geochemical compositions consistent with high-K calc-alkaline series magmatism. The samples display highly fractionated light rare-earth element(REE) distributions and heavy REE distributions with weakly negative Eu anomalies on chondrite-normalized REE patterns. The trace element distributions exhibit positive anomalies for large-ion lithophile elements(Rb, K, U, Th and Pb) and negative anomalies for high-field-strength elements(Nb and Ti) relative to primitive mantlenormalized values. The Eocene quartz diorite porphyry yielded εNd(t) values ranging from-3.6 to-5.2,(~(87)Sr/~(86)Sr)i values in the range 0.7046–0.7063 and initial radiogenic Pb isotopic compositions with ranges of 18.599–18.657 ~(206)Pb/~(204)Pb, 15.642–15.673 ~(207)Pb/~(204)Pb and 38.956–39.199 ~(208)Pb/~(204)Pb. In contrast, the Miocene granitoid plutons yielded ε_(Nd)(t) values ranging from-6.1 to-7.3 and(87Sr/86Sr)i values in the range 0.7071–0.7078 with similar Pb isotopic compositions to the Eocene quart diorite. The Sr-Nd-Pb isotopic compositions of the rocks are consistent with formation from magma containing a component of remelted ancient crust. Zircon grains from the Eocene quartz diorite have ε_(Hf)(t) values ranging from-5.2 to +0.9 and two-stage Hf model ages ranging from 1.07 to 1.46 Ga, while zircon grains from the Miocene granitoid plutons have ε_(Hf)(t) values from-9.9 to +4.2 and two-stage Hf model ages ranging from 1.05–1.73 Ga, indicating that the ancient crustal component likely derives from Paleo- to Mesoproterozoic basement. This source is distinct from that of most porphyry Cu-Mo-Au deposits in the eastern part of the Gangdese porphyry copper belt, which likely originated from juvenile crust. We therefore consider melting of ancient crustal basement to have contributed significantly to the formation Miocene porphyry Cu-Mo-Au deposits in the western part of the Gangdese porphyry copper belt.     

Synkinematic high-K calc-alkaline plutons associated with the Pan-African Central Cameroon shear zone (W-Tibati area): Petrology and geodynamic significance

Four plutons from the W-Tibati area of central Cameroon crop out in close relationships with the Pan-African Adamawa ductile shear zone (Central Cameroon Shear Zone: CCSZ). These plutons include diorites, tonalites, granodiorites and granites, and most of them are porphyritic due to the abundance of pink K-feldspar megacrysts. Syn-kinematic magma emplacement is demonstrated by the elongate shape of the plutons and by magmatic and ductile (gneissic) foliations that strike parallel to or at a low angle with the CCSZ; the foliation obliquity is consistent with dextral transcurrent tectonics. Whole-rock geochemistry points to high-K calc-alkaline to shoshonitic magmatism. Mixing-mingling features can be observed in the field. However, fractional crystallization of plagioclase, amphibole, biotite (+ K-feldspar in the more felsic compositions) appears to have played a dominant role in the magmatic differentiation processes, as confirmed by mass balance calculations based on major elements. Isotopic signatures suggest that the magmas may have originated from different sources, i.e. either from a young mafic underplate for most magmas with ε_Ndi(600 Ma) around −1 to −2 and Sri_(600 Ma) around 0.705, or from an enriched lithospheric mantle for some diorites with ε_Ndi(600 Ma) at −6 and Sri_(600 Ma) at 0.7065; mixing with young crustal component is likely. The plutonic rocks of W-Tibati are similar to other Pan-African high-K calk-alkaline syn-kinematic plutons in western Cameroon. They also display striking similarities with high-K calk-alkaline plutons associated with the Patos and Pernambuco shear zones of the Borborema province in NE Brazil.     

The geochemical and Sr–Nd isotopic characteristics of Eocene to Miocene NW Anatolian granitoids: Implications for magma evolution in a post-collisional setting

Early Eocene to Early Miocene magmatic activity in northwestern Anatolia led to the emplacement of a number of granitoid plutons with convergent margin geochemical signatures. Granitoid plutons in the area are mainly distributed within and north of the suture zone formed after the collision of the Anatolide-Tauride platform with the Pontide belt. We present geochemical characteristics of three intrusive bodies in the region in order to identify their source characteristics and geodynamic significance. Among these, the Çataldağ and Ilıca-Şamlı plutons are located to the north and the Orhaneli pluton is located to the south of the IAESZ (Izmir-Ankara-Erzincan Suture Zone). The plutons are calc-alkaline, metaluminous, and I-type with compositions from granite to monzonite. They display clear enrichments in LILE and LREE and depletions in HFSE relative to N-MORB compositions and have high ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd ratios.The results of theoretical Fractional Crystallization (FC) model show that the samples are affected by fractionation of K-feldspar, plagioclase, biotite and amphibole. Assimilation and Fractional Crystallization (AFC) modeling indicates that the r value, the proportion of variable contamination to fraction, is high, indicating significant crustal contamination in the genesis of granitoid magmas. Combined evaluation of isotopic and trace element data indicates that the granitoids are the products of mantle-derived mafic magmas variably differentiated by simultaneous crustal contamination and fractional crystallization in lower to middle crustal magma chambers in a post-collisional setting.     

Muscovite dehydration melting: Reaction mechanisms,microstructures, and implications for anatexis

Dehydration melting of muscovite in metasedimentary sequences is the initially dominant mechanism of granitic melt generation in orogenic hinterlands. In dry (vapour-absent) crust, muscovite reacts with quartz to produce K-feldspar, sillimanite, and monzogranitic melt. When water vapour is present in excess, sillimanite and melt are the primary products of muscovite breakdown, and any K-feldspar produced is due to melt crystallization. Here we document the reaction mechanisms that control nucleation and growth of K-feldspar, sillimanite, and silicate melt in the metamorphic core of the Himalaya, and outline the microstructural criteria used to distinguish peritectic K-feldspar from K-feldspar grains formed during melt crystallization. We have characterized four stages of microstructural evolution in selected psammitic and pelitic samples from the Langtang and Everest regions: (a) K-feldspar nucleates epitaxially on plagioclase while intergrowths of fibrolitic sillimanite and the remaining hydrous melt components replace muscovite. (b) In quartzofeldspathic domains, K-feldspar replaces plagioclase by K⁺–Na⁺ cation exchange, while melt and intergrowths of sillimanite+quartz form in the aluminous domains. (c) At 7–8 vol.% melt generation, the system evolves from a closed to open system and all phases coarsen by up to two orders of magnitude, resulting in large K-feldspar porphyroblasts. (d) Preferential crystallization of residual melt on K-feldspar porphyroblasts and coarsened quartz forms an augen gneiss texture with a monzogranitic-tonalitic matrix that contains intergrowths of sillimanite+tourmaline+muscovite+apatite. Initial poikiloblasts of peritectic K-feldspar trap fine-grained inclusions of quartz and biotite by replacement growth of matrix plagioclase. During subsequent coarsening, peritectic K-feldspar grains overgrow and trap fabric-aligned biotite, resulting in a core to rim coarsening of inclusion size. These microstructural criteria enable a mass balance of peritectic K-feldspar and sillimanite to constrain the amount of free H₂O present during muscovite dehydration. The resulting modal proportion of K-feldspar in the Himalayan metamorphic core requires vapour-absent conditions during muscovite dehydration melting and leucogranite formation, indicating that the generation of large volumes of granitic melts in orogenic belts is not necessarily contingent on an external source of fluids.     

Thermodynamic analysis of garnet growth zoning in eclogite facies granodiorite from M. Mucrone, Sesia Zone, Western Italian Alps

Samples of the metagranodiorite from M.  Mucrone (Sesia zone, Western Alps) show pseudomorphic and coronitic textures where the igneous minerals were partially replaced by high-pressure metamorphic assemblages. The original magmatic paragenesis consisted of quartz, plagioclase, K-feldspar, biotite and minor phases. During the eclogitic event the original plagioclase was fully replaced by zoisite, jadeite and quartz ± K-feldspar pseudomorphic symplectites and the biotite was in part replaced by phengitic mica. Moreover, a composite corona often developed around the biotite. This corona consists of a layer of phengite I and garnet and, where the igneous biotite and feldspars were in contact, of an outer layer of phengite II intergrown with quartz. Biotite, phengite and K-feldspar are homogeneous while garnet shows a strong composition zoning recording the kinetics of the metamorphic reactions. A numerical simulation of the observed garnet zoning is performed assuming that intercrystalline diffusion and plagioclase resorption were the slowest rate-determining processes during the prograde P-T path. The metamorphic paragenesis constrains the P-T path chosen in the simulation. The comparison between measured and calculated garnet zoning permits evaluation of the relative weights of interface kinetics, grain-boundary and lattice diffusion. Received: 26 November 1997 / Accepted: 6 August 1999     

Petrology and petrogenesis of the Mount Stuart batholith — Plutonic equivalent of the high-alumina basalt association?

The Mount Stuart batholith is a Late Cretaceous calc-alkaline pluton composed of rocks ranging in composition from two-pyroxene gabbro to granite. Quartz diorite is most abundant. This batholith may represent the plutonic counterpart of the high-alumina basalt association. A petrogenetic model is developed in which this intrusive series evolved from one batch of magnesian high-alumina basalt, represented by the oldest intrusive phase, by successive crystal fractionation of ascending residual magma. However, the possibility that this intrusive suite originated from an andésite (quartz diorite) parent by fractionation cannot be excluded.Computer modeling of this intrusive sequence provides a quantitative evaluation of the sequential change of magma composition. These calculations clearly indicate that the igneous suite is consanguineous, and that subtraction of early-formed crystals from the oldest rock is capable of reproducing the entire magma series with a remainder of 2–3% granitic liquid. This model requires that large amounts of gabbroic cumulate remain hidden at depth- an amount equal to approximately 8–10X the volume of the exposed batholith. Mass balances between the amounts of cumulate and residual liquid calculated compare favorably with the observed amounts of intermediate rocks exposed in the batholith, but not with the mafic rocks.Mafic magmas probably fractionated at depth by crystal settling, whereas younger quartz diorite and more granitic magmas underwent inward crystallization producing gradationally zoned plutons exposed at present erosional levels.     

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 morphology and chemistry of K-feldspar megacrysts from Ikizdere Pluton: evidence for acid and basic magma interactions in granitoid rocks, NE-Turkey

Ikizdere Pluton consists of granite, granodiorite, tonalite, monzonite, quartz monzonite containing pinkish colored K-feldspar megacrysts (KFMs). The crystal sizes of the KFMs range from 1 to 4 cm. The lath-shaped megacrysts are uniformly (i.e., randomly) distributed in the host plutonic rocks and have mafic and felsic inclusions whose crystal sizes are smaller than 1 mm. The crystal inclusions are biotite, slightly annitic in composition with X_Mg[=Fe_tot/(Fe_tot+Mg)]=0.50-0.58, amphibole (magnesio-hornblende, X_Mg[=Mg/(Mg+Fe_tot)]=0.70-0.79), iron-titanium oxide (low titanium magnetit and ilmenite), plagioclase (Ab₇₅₋₂₅An₆₅₋₃₅) and as minor quartz. The compositions of the KFMs range from Or₉₅Ab₅An₀ to Or₈₂Ab₁₇An₁. BaO contents of the megacrysts increase from core to rim. The mafic and felsic inclusions are compositionally similar those of the host rocks.The chemical and textural features of K-feldspar are typical for megacrysts that grew as phenocrysts in dynamic granitoidic magma systems. The overgrowth of KFMs and mafic magma injections (magma mixing) may be related to temperature, pressure and compositional fluctuations in the magma chamber. Remnant of earlier formed K-feldspar crystals remain in the felsic magma system, while the mafic injection can decompose some earlier precipitated KFMs. The remnant of K-feldspar remaining after mafic injection are overgrown by rapid diffusion of Ba, K and Na elements in liquid phase, during the later stages of crystallization of the host magma.     

Deciphering the source and contamination history of peraluminous magmas using δ18O of accessory minerals: examples from garnet-bearing plutons of the Sierra Nevada batholith

Peraluminous granitoids provide critical insight as to the amount and kinds of supracrustal material recycled in the central Sierra Nevada batholith, California. Major element concentrations indicate Sierran peraluminous granitoids are high-SiO₂ (68.9–76.9) and slightly peraluminous (average molar Al₂O₃/(CaO + Na₂O + K₂O)=1.06). Both major and trace element trends mimic those of other high-silica Sierran plutons. Garnet (Grt) in the peraluminous plutons is almandine–spessartine-rich and of magmatic origin. Low grossular contents are consistent with shallow (<4 kbar) depths of garnet crystallization. Metasediments of the Kings Sequence commonly occur as wallrocks associated with the plutons, including biotite schists that are highly peraluminous (A/CNK=2.25) and have high whole rock (WR) δ¹⁸O values (9.6–21.8‰, average=14.5±2.9‰, n=26). Ultramafic wallrocks of the Kings–Kaweah ophiolite have lower average δ¹⁸O (7.1±1.3‰, n=9). The δ¹⁸O(WR) of the Kings Sequence is variable from west to east. Higher δ¹⁸O values occur in the west, where quartz in schists is derived from marine chert; values decrease eastward as the proportion of quartz from igneous and metamorphic sources increases. Peraluminous plutons have high δ¹⁸O(WR) values (9.5–13‰) consistent with supracrustal enrichment of their sources. However, relatively low initial ⁸⁷Sr/⁸⁶Sr values (0.705–0.708) indicate that the supracrustal component in the source of peraluminous magmas was dominantly altered ocean crust and/or greywacke. Also, plutons lack or have very low abundances (<1% of grains) of inherited zircon (Zrc) cores. Average δ¹⁸O(Zrc) is 7.9‰ in peraluminous plutons, a higher value than in coeval metaluminous plutons (6–7‰). Diorites associated with peraluminous plutons also have high δ¹⁸O(Zrc), 7.4–8.3‰, which is consistent with the diorites being derived from a similar source. Magmatic garnet has variable δ¹⁸O (6.6–10.5‰, avg.=7.9‰) due to complex contamination and crystallization histories, evidenced by multiple garnet populations in some rocks. Comparison of δ¹⁸O(Zrc) and δ¹⁸O(Grt) commonly reveals disequilibrium, which documents evolving magma composition. Minor (5–7%) contamination by high δ¹⁸O wallrocks occurred in the middle and upper crust in some cases, although low δ¹⁸O wallrock may have been a contaminant in one case. Overall, oxygen isotope analysis of minerals having slow oxygen diffusion and different times of crystallization (e.g., zircon and garnet), together with detailed textural analysis, can be used to monitor assimilation in peraluminous magmas. Moreover, oxygen isotope studies are a valuable way to identify magmatic versus xenocrystic minerals in igneous rocks. Electronic Supplementary Material Supplementary material is available for this article at     

Sm–Nd geochronology and geochemistry of a Neoproterozoic gabbro in the Kuluketage block,north-western China

In this article, we report whole-rock and mineral Sm–Nd isotopic and whole-rock elemental and Sr–Nd isotopic data of Xingdi No. 1 mafic–ultramafic intrusion in the western Kuluketage block, north-eastern Tarim. Xingdi No. 1 mafic–ultramafic intrusion is the largest in the Xingdi mafic–ultramafic belt, with an exposed area of ca. 20 km². It intruded into the Palaeoproterozoic basement. Gabbro is the major rock type and there is minor olivine pyroxenite. Sm–Nd geochronometry of the gabbro gives an isochron age of 761.2 ± 31.2 million years, identical to the intrusive age of Xingdi No. 2 pluton (760 ± 6 million years). The gabbro is systematically enriched in large ion lithosphile elements and light rare earth elements and depleted in high field strength elements and heavy rare earth elements. The studied rocks are characterized by low whole-rock and mineral ?Nd_(t) values (?7.8 to??7.1) and elevated (⁸⁷Sr/⁸⁶Sr) _i values (0.7066–0.7073). These geochemical characteristics, together with the presence of abundant hornblende, biotite, bladed biotite enclosed in amphibole, and crescent-shaped Palaeoproterozoic wall-rock xenoliths in the intrusion, are key features of magma mixing in the source or assimilation during its emplacement. The rocks have a Zr/Y ratio of 3.81–13, which falls in the within-plate basalt area. As Xingdi No. 1 and No. 2 plutons formed at the same period and display similar geochemical characteristics, we propose that they formed within the same tectonic setting and were derived from the same source, but No. 1 pluton experienced a higher extent of evolution and contamination. Previous studies have shown that the Neoproterozoic tectonic and magmatic events in Kuluketage comprise syn-collisional granite around TC (ca. 1.0–0.9 Ga), post-collisional K-rich granite and alkaline mafic–ultramafic intrusions (ca. 830–800 Ma), and rifting-related mafic–ultramafic plutons, dikes, and bimodal volcanic rocks (ca. 774–744 Ma).     

Granitic magmas with I-type affinities,from mainly metasedimentary sources: the Harcourt batholith of southeastern Australia

The high-K, calcalkaline granitic rocks of the 370 Ma, post-orogenic Harcourt batholith in southeastern Australia have I-type affinities but are mildly peraluminous and have remarkably radiogenic isotope characteristics, with ⁸⁷Sr/⁸⁶Sr_t in the range 0.70807 to 0.714121 and εNd_t in the range ??5.6 to ??4.3. This batholith appears to be a good example of magmas that were derived through partial melting of distinctly heterogeneous source rocks that vary from intermediate meta-igneous to mildly aluminous metasedimentary rocks, with the balance between the two rock types on the metasedimentary side. Such transitional S-I-type magmas, formed from mainly metasedimentary source rocks, may be more common than is generally realised. The Harcourt batholith also contains mainly granodioritic igneous microgranular enclaves (IMEs). Like their host rocks, the IMEs are peraluminous and have rather radiogenic isotope signatures (⁸⁷Sr/⁸⁶Sr_t of 0.71257–0.71435 and εNd_t of ??7.3 to ??4.3), though some are hornblende-bearing. Origins of these IMEs by mixing a putative mantle end member with the host granitic magma can be excluded because of the variability in whole-rock isotope ratios and, for the same reason, the IME magmas cannot represent quench cumulates (autoliths) from the host magmas. Less abundant monzonitic to monzosyenitic IMEs cannot represent accumulations of magmatic biotite and/or alkali feldspar because K-feldspar is absent, and there is no co-enrichment of K₂O and FeO?+?MgO, nor can they be mixtures of anything plausible with the host-rock magma. The granodioritic IMEs probably originated through high degrees of assimilation of a range of crustal materials (partial melts?) by basaltic magmas in the deep crust, and the monzonitic IMEs as melts of enriched subcontinental mantle. Such enclave suites provide little or no information on the chemical evolution of their host granitic rocks.     

Sequential melting and fractional crystallization: Granites from Guarda-Sabugal area, central Portugal

Seven distinct phases of Variscan two-mica granite are recognized in the Guarda-Sabugal area. They intruded the Cambrian schist-metagraywacke complex, crystallized in the middle crust, and are syn- to late-D3 (309.2 ± 1.8 Ma), late-D3 (304–300 Ma) and late- to post-D3 (299 ± 3 Ma; ID-TIMS ages on zircon and monazite). Two of the granites, G2 and G5, are close in age and have similar Sr, Nd and O isotope characteristics but contrasting whole rock and mineral features and formed by sequential increasing degree of partial melting of a common metasedimentary protolith. During sequential melting Ti, total Fe, Mg, Ca, Zr, Zn, Sr, Ba and REE contents and (La/Yb)_N increase and Si and Rb contents decrease, plagioclase becomes richer in anorthite and biotite and muscovite richer in Ti and Mg. Each of these granites evolved subsequently by fractional crystallization of quartz, K-feldspar, plagioclase, biotite and ilmenite, defining separate series G2–G3–G7 and G5–G6 containing late Sn-bearing differentiates. Two other granites G1 and G4 represent distinct pulses of magma with individual fractionation trends for major and trace elements and distinct (⁸⁷Sr/⁸⁶Sr)₃₀₀, ?Nd₃₀₀ and δ¹⁸O values.     

The Bjerkreim-Sokndal Layered Intrusion, Rogaland, SW Norway: Evidence from marginal rocks for a jotunite parent magma

The Late-Proterozoic Bjerkreim-Sokndal Layered Intrusion (BKSK) consists of andesine anorthosite, leuconorite, troctolite, norite, gabbronorite, jotunite, mangerite, quartz mangerite and charnockite. The sequence of appearance of cumulus minerals and their compositions suggest a parent magma that was evolved, had plagioclase (±olivine) on the liquidus, was sufficiently TiO₂-rich for hemo-ilmenite to crystallise early, and low in CaO and CaO/Al₂O₃compared to basalts as reflected by the sodic plagioclases and the delayed appearance of cumulus augite. Fine- to medium-grained jotunites found along the northern contact of the BKSK consist of plagioclase (An_45–53), inverted pigeonite (Mg# = 55-50), sparse augite (Mg# = 69-59), Fe-Ti oxides, K-feldspar, quartz and apatite. They are basic to intermediate rocks with relatively high FeO^total, high TiO₂, low MgO/MgO + FeO, moderate Al₂O₃ and low CaO and normative diopside. The jotunites have compositions that are consistent with the parental magma for the lower part of the BKSK Layered Series, and are interpreted as being marginal chills. Similar, but slightly more differentiated, jotunite magmas were subsequently emplaced into the BKSK and the surrounding region as broad dykes and small plutons. Jotunite is a minor rock type in most massif-type anorthosite provinces but may have an important petrological significance.