Composite Pluton Research Articles

Establishing an Indosinian geochronological framework for episodic granitic emplacement and W-Sn-Nb-Ta mineralization in Limu mining district, South China

The Limu mining district is located in the Nanling Polymetallic Belt in South China. The district contains a number of W-Sn-Nb-Ta deposits and a number of exposed and unexposed granitic stocks that are connected as a single pluton at depth, referred to as Limu composite pluton. The deposits occur either within or above the stocks of the pluton, bearing a close mutual spatial relationship. Two types of W-Sn-Nb-Ta mineralization or ores exist in the district: the predominant quartz-vein type and the lesser albite-granite type. Extensive high-precision U-Pb zircon (SHRIMP and LA-ICP-MS) and Ar-Ar muscovite geochronological analyses on the granite stocks and the ores were performed in recent years, with exception only for the quartz-vein-type ores. This study applied Ar-Ar muscovite dating method for the quartz-vein-type ores at five quartz-vein-type deposits and obtained five new 40Ar/39Ar plateau ages ranging from 210 ± 1.5 Ma to 205.5 ± 1.6 Ma. New U-Pb zircon dating with LA-ICP-MS method was also performed for the non-mineralized Paoshuiling and mineralized Yucai stocks, which yielded weighted mean ages of 230.8 ± 0.63 Ma and 222.7 ± 1.2 Ma, respectively. With reliable geochronological data available for all the granites and mineralization types, combining field and petrographic observations, this study attempts to establish a high-resolution geochronological framework for three episodic granitic stocks and two mineralization types of the Limu district. The first episode is represented by Paoshuiling annite-lepidolite granite aged at 230.8 ± 0.63 Ma; the second-episode granite was emplaced at 224.8 ± 1.6–222.7 ± 1.2 Ma; and the third at 218.3 ± 2.4–212.3 ± 1.8 Ma. The high-temperature albite-granite-type mineralization began in the second episode of granitic emplacement and lasted into the third episode. The quartz-vein-type mineralization was subsequent to the albite-granite-type mineralization and immediately dominated the post-third-episode hydrothermal ore-forming process at 210 ± 1.5–205.5 ± 1.6 Ma as revealed by new muscovite Ar-Ar ages obtained in this study.

Genesis of magma mixing and mingling of the Awengcuo composite plutons in western segment of Bangongco-Nujiang Suture Zone: Evidence from geochemistry, geochronology and mafic microgranular enclaves (MMEs)

阿翁错复式岩体位于班公湖-怒江缝合带西段，是班公湖-怒江特提斯洋俯冲消减，造山过程中岩浆响应的重要组成部分，以广泛发育暗色微粒包体和岩浆混合、不协调现象为特征。本文以阿翁错复式岩体为研究对象，对寄主岩和暗色微粒包体开展了系统的地质学、地球化学和锆石U-Pb年代学研究，探讨了阿翁错复式岩体的岩浆混合成因。暗色微粒包体塑性变形特征明显，与寄主岩呈截然或渐变接触，偶见反向脉发育，包体具细-中粗粒结构，含斜长石、钾长石、角闪石、暗色镶边石英等斑晶，偶见角闪石斑晶横跨包体和寄主岩，在包体及包体周围寄主岩中见长柱状斜长石、角闪石和针状磷灰石等结构特征，表明暗色微粒包体为岩浆混合作用的产物。寄主岩与包体均为准铝质、钙碱性-高钾钙碱性系列岩石，主要氧化物含量在Harker图解上具有良好的线性关系，稀土元素配分曲线图和微量元素蛛网图具有高度一致性，表明二者具有强烈的地球化学亲源关系，且经历了相似的岩浆演化过程。寄主岩和暗色微粒包体的成岩年龄分别为109.1±1.0Ma和107.4±0.7Ma，岩浆混合作用发生在早白垩世晚期，处于班公湖-怒江特提斯洋由弧-陆碰撞向陆陆碰撞的转换阶段即软碰撞阶段。研究表明，在班公湖-怒江特提斯洋向北向羌塘地块之下俯冲的背景下，洋壳脱水，引起上覆地幔楔发生部分熔融，形成镁铁质岩浆，镁铁质岩浆向上运移，并底侵于壳-幔边界，引发下地壳物质发生部分熔融，形成长英质岩浆，当镁铁质岩浆从底部注入长英质岩浆房时，镁铁质岩浆快速冷凝，形成部分色率高、粒度细，具冷凝边的包体，与寄主岩呈截然型接触，随着端元岩浆之间的温差逐渐降低，包体色率降低，粒度变大，与寄主岩呈渐变过渡。

Tectonic strain recorded by magnetic fabrics (AMS) in plutons, including Mt Kinabalu, Borneo: A tool to explore past tectonic regimes and syn-magmatic deformation

Tectonic strain commonly overprints magmatic fabrics in AMS (Anisotropy of Magnetic Susceptibility) data for plutonic rocks produced by both compressional and extensional regimes. Mt Kinabalu, Borneo, is a composite pluton with an exceptional vertical range of exposure and clearly defined internal contacts. We show that tectonic fabrics are recorded pervasively throughout the intrusion, even near contacts, and present a workflow distinguishing compressive and extensional syn-magmatic deformation. At Mt Kinabalu this reveals a pervasive tectonic fabric indicating NW-SE Miocene extension in Borneo at 7.9–7.3 Ma, later than previously recognised, oriented NW-SE at 319° ±13.1°. Comparing data from Mt Kinabalu with data from globally distributed studies shows that tectonic strain is commonly recorded by plutons. Therefore, AMS fabric can be used to identify the syn-magmatic tectonic setting and combined with both geochronology and evidence for paleomagnetic rotation to provide a powerful tool for accurate determination of syn-magmatic tectonic regimes and strain orientations within temporal frameworks.

Timing of the final closure of the Irtysh–Zaysan Ocean: New insights from the earliest stitching pluton in the northern West Junggar, NW China

A tight constraint on the closure time of the Irtysh–Zaysan Ocean, a major southwestern branch of the Paleo‐Asian Ocean, is crucial for understanding the final amalgamation of the Central Asian Orogenic Belt; however, no agreement has been reached. This study presents new zircon U–Pb ages and whole‐rock geochemical data for the Keluke composite pluton in the Zharma–Saur arc of the northern West Junggar (NW China). The Keluke composite pluton mainly includes the hornblende gabbro, gabbroic diorite, and K‐feldspar granite. The laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating indicates that the hornblende gabbro and K‐feldspar granite formed at 334 ± 1 Ma and 319 ± 4 Ma, respectively. The gabbroic and dioritic rocks are calc‐alkaline and arc‐related plutonism characterized by strong depletions in Nb–Ta–Ti, thus being the product of the subduction of the Irtysh–Zaysan Ocean. By contrast, the K‐feldspar granite is geochemically similar to the post‐collisional highly fractionated I‐type granite. In particular, when compared with regional magmatic events, the Keluke K‐feldspar granite represents the first stitching plutonism in response to the termination of the Irtysh–Zaisan Ocean, which strongly indicates that the Irtysh–Zaisan Ocean was closed prior to the earliest late Carboniferous.

Pluton construction and deformation in the Sveconorwegian crust of SW Norway: Magnetic fabric and U-Pb geochronology of the Kleivan and Sjelset granitic complexes

The Kleivan and Sjelset granitic complexes are two composite plutons, containing both orthopyroxene and biotite (± hornblende) facies, emplaced in the Sveconorwegian (Grenvillian) high-grade basement of SW Norway. A structural study of these two plutons, based on the anisotropy of magnetic susceptibility (AMS) technique and combined with high-precision U-Th-Pb zircon dating, is presented here. Geochronological data demonstrate a rapid emplacement of successive magmatic pulses in the Kleivan complex (from 936.94 ± 0.42 Ma to 935.62 ± 0.67 Ma) and, on the contrary, a non-negligible rest period (∼3.2 Ma) in the construction of the Sjelset complex that was formed by two pulses emplaced, respectively, at 935.67 ± 0.37 Ma and 932.43 ± 0.75 Ma. Locally discordant magnetic fabrics in the latter pluton confirm this rather protracted construction time. Thermomagnetic and hysteresis measurements supporting the AMS data indicate a magnetic mineralogy dominated by a multidomain, Ti-poor titanomagnetite, except in samples having a very low magnetic susceptibility. The susceptibility magnitudes, paramagnetic to ferromagnetic in agreement with the rock magnetic data, rely on the petrographic rock-types and on the alteration degree. Image analysis confirms that the magnetic fabric is usually coaxial with the shape fabric in both complexes, supporting the use of AMS as a proxy for the petrofabric orientation. Combined with micro- and macrostructural data, the magnetic fabric demonstrates that the Kleivan and Sjelset granitic complexes have their internal fabrics largely dominated by tectonic strain. Models of synfolding emplacement coeval with the last stage of Sveconorwegian contraction recorded in the area are proposed for the two plutons.

Evaluating the importance of metamorphism in the foundering of continental crust

The metamorphic conditions and mechanisms required to induce foundering in deep arc crust are assessed using an example of representative lower crust in SW New Zealand. Composite plutons of Cretaceous monzodiorite and gabbro were emplaced at ~1.2 and 1.8 GPa are parts of the Western Fiordland Orthogneiss (WFO); examples of the plutons are tectonically juxtaposed along a structure that excised ~25 km of crust. The 1.8 GPa Breaksea Orthogneiss includes suitably dense minor components (e.g. eclogite) capable of foundering at peak conditions. As the eclogite facies boundary has a positive dP/dT, cooling from supra-solidus conditions (T > 950 ºC) at high-P should be accompanied by omphacite and garnet growth. However, a high monzodioritic proportion and inefficient metamorphism in the Breaksea Orthogneiss resulted in its positive buoyancy and preservation. Metamorphic inefficiency and compositional relationships in the 1.2 GPa Malaspina Pluton meant it was never likely to have developed densities sufficiently high to founder. These relationships suggest that the deep arc crust must have primarily involved significant igneous accumulation of garnet–clinopyroxene (in proportions >75%). Crustal dismemberment with or without the development of extensional shear zones is proposed to have induced foundering of excised cumulate material at P > 1.2 GPa.

Geochemistry and zircon U–Pb–Hf isotopes of the granitoids of Qianjinchang pluton in the Xi Ujimqi, Inner Mongolia: Implications for petrogenesis and geodynamic setting

We conducted zircon U–Pb dating and geochemical analyses for the Qianjinchang (QJC) pluton in the Xi Ujimqi, Northeast China, with an aim to determine their ages, petrogenesis, and sources. The QJC pluton consists of coarse/medium‐grained biotite monzogranite in the core and fine‐grained biotite granodiorite in the rim; those rocks intrude into quartz diorite but are intruded by minor intrusive phases, including small biotite syenogranite, diorite bodies, late diorite, granodiorite, granite, and pegmatite dykes. Our new laser ablation inductively coupled plasma mass spectrometry zircon U–Pb data indicate that the QJC composite pluton composed of 2 phases of magmatic activities, with the ages of 301~313 Ma for the quartz diorite, 283 ± 1 Ma for the biotite granodiorite, 280 ± 1 Ma for the biotite syenogranite, and 280 ± 2 Ma for the diorite dyke. Hf isotopic analyses for the quartz diorite sample show εHf(t) = 3.75 to 11.72, with 2‐stage Hf model age (TDM2) ranging 568–1,078 Ma. The biotite syenogranite sample also shows a depleted εHf(t) = 4.47 to 8.71, withTDM2ranging 745–1,015 Ma, suggesting the major involvement of juvenile crustal components. The various εHfvalues of the QJC pluton indicate a hybrid magma source of juvenile material with old crustal component, and theTDM2values increase from the Carboniferous to Permian, which suggests an increasing proportion of old continental material during this period. Petrological and geochemical characteristics of the biotite granodiorite and biotite monzogranite samples suggest that they are S‐type granites and derived from partial melting of the clay‐poor, plagioclase‐rich psammitic source, produced at low‐medium pressure. The biotite syenogranite sample belongs to alkaline and shoshonitic series and probably formed by a hybridization process between basaltic magma and old continental components. Combined with previous studies on the contemporaneous magma‐tectonic activities in the Xilinhot microcontinent, we suggest that the QJC pluton formed in a postcollisional setting.

Nature and origin of Triassic igneous activity in the Western Qinling Orogen: the Wenquan composite pluton example

ABSTRACTThe Western Qinling has been acknowledged to witness superimposed orogeny including north subduction of Paleotethys ocean and collision between North China and South China blocks; however, the precise timing constraints on transition of tectonic regime are remaining enigmatic. The Wenquan composite batholith comprising five phases and mafic enclaves is an ideal example to unlock this puzzle. The host granitoids are felsic, metaluminous to peraluminous, and high-K calc-alkaline to shoshonitic suite with I-type affinity. The mafic enclaves, however, are intermediate, and high-K calc-alkaline to shoshonitic. Zircon ages of multiple phases indicate an episodic growth lasting nearly 30 million years ranging from 238, 228, 218 to 208 Ma, consistent to Triassic igneous activity recording a transition regime from a subduction setting to a syn-collision setting and a post-collision setting in Western Qinling. Lead isotopes of whole-rock and K-feldspar at Wenquan and Lu-Hf isotopes of zircons separated from biotite monzogranite porphyry, porphyritic monzogranite, monzogranite porphyry, and hosted mafic enclaves suggest that the heat and the hot mafic melt initiated by the break-off of the northward subducting South China block lithosphere triggered partial melting of the Mesoproterozoic subcontinental lithospheric mantle to produce mafic magmas, and the underplated mafic magmas caused partial melting of the shallow subducted Mesoproterozoic lower crust generating granitic magmas at Wenquan. Combined our field observations and petrology study with a holistic review on previous geochronological and geochemical data of Triassic granitoids throughout the Western Qinling, we in this contribution proposed that the Triassic igneous activity in the Western Qinling corresponding to superimposed orogeny evolved from the northward subduction of Palaeotethys ocean (250–235 Ma) through syn-collision (228–215 Ma) to post-collision (215–185 Ma) between the North China and South China blocks.

Mineralogical, geochemical and Sr-Nd isotopes characteristics of fluorite-bearing granites in the Northern Arabian-Nubian Shield, Egypt: Constraints on petrogenesis and evolution of their associated rare metal mineralization

The Central Eastern Desert (CED) of Egypt, a part of Neoproterozoic Arabian Nubian Shield (ANS), embraces a multiplicity of rare metal bearing granitoids. Gabal El-Ineigi represents one of these granitic plutons and is a good example of the fluorite-bearing rare metal granites in the ANS. It is a composite pluton consisting of a porphyritic syenogranite (SG; normal granite) and coarse- to medium-grained highly evolved alkali-feldspar granite (AFG; fluorite and rare metal bearing granite) intruded into older granodiorite and metagabbro-diorite rocks. The rock-forming minerals are quartz, K-feldspar (Or94-99), plagioclase (An0-6) and biotite (protolithonite-siderophyllite) in both granitic types, with subordinate muscovite (Li-phengite) and fluorite in the AFG. Columbite-(Fe), fergusonite-(Y), rutile, zircon and thorite are the main accessory phases in the AFG while allanite-(Ce) and epidote are exclusively encountered in the SG. Texture and chemistry of minerals, especially fluorite, columbite and fergusonite, support their magmatic origin. Both granitic types are metaluminous to weakly peraluminous (A/CNK=0.95–1.01) and belong to the post-collisional A2-type granites, indicating melting of underplated mafic lower crust. The late phase AFG has distinctive geochemical features typical of rare metal bearing granites; it is highly fractionated calc-alkaline characterized by high Rb, Nb, Y, U and many other HFSE and HREE contents, and by extremely low Sr and Ba. Moreover, the REE patterns show pronounced negative Eu anomalies (Eu/Eu∗=0.03 and 0.06) and tetrad effect (TE1,3=1.13 and 1.27), implying extensive open system fractionation via fluid–rock interactions that characterize the late magmatic stage differentiation. The SG is remarkably enriched in Sr, Ba and invariably shows a relative enrichment in light rare-earth elements (LREEs). The SG rocks (569±15Ma) are characterized by relatively low initial 87Sr/86Sr ratios (0.7034–0.7035) that suggest their derivation from the mantle, with little contamination from the older continental crust. By contrast, the AFG has very high 87Rb/86Sr and 87Sr/86Sr ratios that reflect the disturbance of the Rb-Sr isotopic system and may give an indication for the high temperature magma-fluid interaction. The positive εNd(t) values of AFG (+7.40) and SG (+5.17), corresponding to young Nd-TDM2 ages ranging from 707 to 893 Ma, clearly reflect the juvenile crustal nature of Gabal El-Ineigi granitoids and preclude the occurrence of pre-Neoproterozoic continental crust in the ANS. The field relationships, chemical, petrological and isotopic characteristics of El-Ineigi SG and AFG prove that they are genetically not associated to each other and indicate a complex origin involving two compositionally distinct parental magmas that were both modified during magmatic fractionation processes. We argue that the SG was formed by partial melting of a mid-crustal source with subsequent fractional crystallization. In contrast, the AFG was generated by partial melting and fractionation of Nb- and Ta-rich amphibole (or biotite) of the lower crust. The appreciable amounts of fluorine in the magma appears to be responsible for the formation of rare metal element complexes (e.g., Nb, Ta, Sn and REEs), and could account for the rare metal mineralization in the El-Ineigi AFG.

Mantle influence of syn- to late-Grenvillian alkaline magmatism in the Grenville Province: causes and implications

Several features of Geon 10 magmatic evolution in the Grenville Orogenic Belt is difficult to reconcile with generally accepted models of protracted (ca. 100 Myr) continent–continent collision during the Grenville Orogeny. Particularly the presence of (partly) mantle-derived magmatic rocks, some with subduction signatures, intruded during the inferred climax of the orogeny, is not well accounted for in existing models. We present new geochemical, Lu–Hf isotopic and U–Pb geochronological data from three alkaline composite plutons in Quebec, Canada, that give important clues to the tectono-magmatic evolution from ca. 1040 to 1000 Ma of the Grenville Laurentian margin. The oldest pluton, emplaced at ca. 1038 Ma, has a geochemical composition compatible with derivation in an arc setting by partial melting of subcontinental lithospheric mantle. The two youngest plutons, emplaced at ca. 1014 and 1009 Ma, respectively, have typical within-plate geochemical signatures showing no obvious influence of subduction. The new and existing data indicate that much of the Grenville Laurentian margin experienced ensialic magmatism through large parts of Geon 10, an observation calling for alternative models to the existing to explain the Geon 10 evolution of the Grenville Orogenic Belt. We propose a model where Ottawan metamorphism and magmatism resulted from accretion of terranes and continued subduction beneath Laurentia until late Geon 10. Late Geon 10 magmatism could have been related to slab breakoff prior to or at the onset of a late Geon 10 collisional event, which has also been recently proposed based on paleomagnetic arguments.

Fractionation of Li, Be, Ga, Nb, Ta, In, Sn, Sb, W and Bi in the peraluminous Early Permian Variscan granites of the Cornubian Batholith: Precursor processes to magmatic-hydrothermal mineralisation

The Early Permian Variscan Cornubian Batholith is a peraluminous, composite pluton intruded into Devonian and Carboniferous metamorphosed sedimentary and volcanic rocks. Within the batholith there are: G1 (two-mica), G2 (muscovite), G3 (biotite), G4 (tourmaline) and G5 (topaz) granites. G1–G2 and G3–G4 are derived from greywacke sources and linked through fractionation of assemblages dominated by feldspars and biotite, with minor mantle involvement in G3. G5 formed though flux-induced biotite-dominate melting in the lower crust during granulite facies metamorphism. Fractionation enriched G2 granites in Li (average 315ppm), Be (12ppm), Ta (4.4ppm), In (74ppb), Sn (18ppm) and W (12ppm) relative to crustal abundances and G1 granites. Gallium (24ppm), Nb (16ppm) and Bi (0.46ppm) are not significantly enriched during fractionation, implying they are more compatible in the fractionating assemblage. Sb (0.16ppm) is depleted in G1-G2 relative to the average upper and lower continental crust. Muscovite, a late-stage magmatic/subsolidus mineral, is the major host of Li, Nb, In, Sn and W in G2 granites. G2 granites are spatially associated with W-Sn greisen mineralisation.Fractionation within the younger G3–G4 granite system enriched Li (average 364ppm), Ga (28ppm), In (80ppb), Sn (14ppm), Nb (27ppm), Ta (4.6ppm), W (6.3ppm) and Bi (0.61ppm) in the G4 granites with retention of Be in G3 granites due to partitioning of Be into cordierite during fractionation. The distribution of Nb and Ta is controlled by accessory phases such as rutile within the G4 granites, facilitated by high F and lowering the melt temperature, leading to disseminated Nb and Ta mineralisation. Lithium, In, Sn and W are hosted in biotite micas which may prove favourable for breakdown on ingress of hydrothermal fluids. Higher degrees of scattering on trace element plots may be attributable to fluid–rock interactions or variability within the magma chamber. The G3-G4 system is more boron-rich, evidenced by a higher modal abundance of tourmaline. In this system, there is a stronger increase of Sn compared to G1-G2 granites, implying Sn in tourmaline-dominated mineral lodes may represent exsolution from G4 granites.G1–G4 granite abundances can be accounted for by 20–30% partial melting and 10–40% fractionation of a greywacke source. G5 granites are analogues of Rare Metal Granites described in France and Germany. These granites are enriched in Li (average 1363ppm), Ga (38ppm), Sn (21ppm), W (24ppm), Nb (52ppm) and Ta (15ppm). Within G5 granites, the metals partition into accessory minerals such as rutile, columbite-tantalite and cassiterite, forming disseminated magmatic mineralisation. High observed concentrations of Li, In, Sn, W, Nb and Ta in G4 and G5 granites are likely facilitated by high F, Li and P, which lower melt temperature and promote retention of these elements in the melt, prior to crystallisation of disseminated magmatic mineralisation.

Feldspar megacrysts from the Santa Angélica composite pluton — Formation/transformation path revealed by combined CL, Raman and LA-ICP-MS data

The studied feldspar megacrysts from the Santa Angélica hybrid rock unit feature complex growth morphologies and geochemical compositions. Early crystallization formed zoned K–Na alkali-feldspar and unzoned oligoclase–andesine. The chemical composition of the zoned alkali-feldspar reflects crystallization in contact with different magma batches, and the chemical composition of the plagioclase indicates growth from a homogeneous magma. Further feldspar development produced mineral chemistry and growth morphology patterns (in both alkali feldspar and plagioclase) that indicate a multi-stage process caused by the combination of chaotic mixing and replenishment. Alkali feldspar and plagioclase megacrysts show dissolution and regrowth textures. Their compositional fields are not separated. Different crystallized and recrystallized domains pass smoothly from one to another. A multiple reequilibration process limited the ordering of the feldspar domain structure. A younger generation of feldspar strongly enriched in K and Na grew over the dissolved margins of the alkali feldspar and plagioclase megacrysts, filling all embayments. Some domains of the alkali feldspar megacrysts recrystallized, resulting in a K- and Ba-rich composition. The trace element patterns of these domains define relatively consistent trends, which may indicate a new stage of equilibrium. In turn, the trace element patterns of previously crystallized and recrystallized domains are irregular, pointing to blurring of the original compositional relationship. Some voids in the megacrysts, formed due to dissolution, are cemented with quartz. The sequential early to late crystallization, recrystallization, and alteration processes are clearly revealed by Polytopic Vector Analysis (PVA). This analysis shows the changes in the alkali-feldspar and plagioclase compositions in two different directions: an initial shift from megacryst crystallization environment towards alkali-rich magma and a second shift towards silica-rich magma. The recognition of the whole sequence of the processes is possible due to the preserved relics of each phase in different feldspar domains.

Internal structure and emplacement mechanism of composite plutons: evidence from Mt Kinabalu, Borneo

The internal structure and emplacement mechanisms of composite plutons are investigated using new field data from the composite Late Miocene granitic intrusion of Mt Kinabalu in northern Borneo. The pluton was emplaced in the upper to middle crust in the Late Miocene at the contact between the ultramafic basement and sedimentary cover rocks. Structural data indicate that emplacement occurred during regional NNW–SSE-oriented extension, challenging tectonic models that involve contemporaneous regional compression. The six major units forming the pluton were accommodated by upward flexure of the cover rocks with most magma pulses emplaced successively beneath their predecessors. However, the irregular 3D internal structure of the pluton also reflects preferential emplacement of successive units along the granite–country rock contact of previous units in preference to the basement–cover rock contact exploited by the initial units. This study highlights the complex emplacement mechanisms and internal structure of composite intrusions and assesses how they differ from models of tabular emplacement.

On the role of contamination and hybridism in formation of the composite Raumid pluton granites (Pamir Mnts.): Results of Sm–Nd Isotope Study

This work presents isotope Sm-Nd data obtained for bulk samples of granites of all 8 emplacement phases of the Raumid granite massif, which occurred 35 Ma ago at a hypabyssal depth during the orogenic stage of development of Southern Pamir fold system. The 147Sm/144Nd ratio in studied collection of granite samples ranges between 0.091 and 0.323; the eNd(T) value is–4.0. The Sm-Nd isotope study results suggest that all granite varieties distinguished in the Raumid massif are comagmatic formations and contamination and hybridization processes did not play any role in REE distribution in granites. At this, the source of parental magma did not change during granite generation. We assume that the only process, resulted in the trace element evolution in granites, was differentiation of three batches of magma sequentially uplifted from the source.

Geochronology, geochemistry and tectonic implications of Weitingchagan composite pluton in northern segment of the Xing‐Meng Orogenic Belt

We undertook zircon U–Pb dating and geochemical analyses of the Weitingchagan (WT) pluton in the Dong Ujimqi area, Northeast China, with an aim of determining their ages, petrogenesis and sources, which are important for understanding the Late Palaeozoic tectonic evolution of the Xing‐Meng Orogenic Belt. The WT pluton consists of coarse‐ to medium‐grained porphyritic granites in the core and medium‐ to fine‐grained monzogranites in the rim, contains abundant microgranular enclaves and is intruded by small amounts of muscovite monzogranites. The results of LA‐ICP‐MS zircon U–Pb dating indicate that the WT composite pluton formed during the Late Carboniferous–Early Permian with the ages ranging from 289 Ma to 312 Ma. Petrological and geochemical characteristics of the WT pluton were derived from the partial melting of crustal materials, and the AFC process played an important role in the magmatic evolution. The porphyritic granites are I‐type granite, and the monzogranites and muscovite monzogranites are highly fractionated I‐type granite. Combined with previous studies on the contemporaneous magma‐tectonic activities in the Uliastai Continental Margin, the study suggests that the WT pluton formed in an extensional environment, which may relate to the closure of the Palaeo‐Asian Ocean. Copyright © 2016 John Wiley & Sons, Ltd.

Temporal and genetic link between incremental pluton assembly and pulsed porphyry Cu-Mo formation in accretionary orogens

Abstract Economically important porphyry Cu-Mo deposits (PCDs) are generally hosted by upper-crustal plutons of variable chemical compositions related to distinct geodynamic settings. The absolute timing and duration of pluton assembly and PCD formation are critical to understanding the genetic relationship between these interrelated processes. Here, we present new comprehensive zircon U-Pb and molybdenite Re-Os ages that tightly constrain the timing and duration of pluton assembly and the age of mineralization in one of the largest ore-bearing plutons of the central Tethyan metallogenic belt, the Meghri-Ordubad pluton, southern Armenia and Nakhitchevan, Lesser Caucasus. This composite pluton was incrementally assembled during three compositionally distinct magmatic episodes over ∼30 m.y., comprising Middle Eocene (48.9–43.1 Ma) calc-alkaline subduction-related magmatism lasting 5.8 ± 0.8 m.y., followed by postsubduction Late Eocene–Middle Oligocene (37.8–28.1 Ma) shoshonitic magmatism over 9.7 ± 0.9 m.y., and Late Oligocene–Early Miocene (26.6–21.2 Ma) adakitic magmatism consisting of shoshonitic dikes and high-K calc-alkaline granodioritic magmas emplaced over 5.4 ± 0.4 m.y. Despite the distinct geodynamic settings and magma compositions, each intrusive suite culminated in the formation of variably sized PCDs, including the giant Oligocene Kadjaran porphyry Cu-Mo deposit associated with high-Sr/Y shoshonitic magmas. Complementary in situ zircon hafnium (εHfzircon = +8 to +11.3) and oxygen (δ18Ozircon = +4.6‰ to +6.0‰) isotope data support a mantle-dominated magma source with limited crustal contribution and/or cannibalization of young and juvenile lower-crustal cumulates. We conclude that, independent of geodynamic setting and magma composition, long-lived (5–10 m.y.) incremental mantle-derived magmatism is a prerequisite to form fertile magmatic-hydrothermal systems, and especially giant PCDs.

Origin of the Late Jurassic to Early Cretaceous peraluminous granitoids in the northeastern Hunan province (middle Yangtze region), South China: Geodynamic implications for the Paleo-Pacific subduction

The Late Mesozoic granitic belt in the northeastern Hunan province (situated in the south of the middle Yangtze region) represents the western front of the large magmatic province of SE China. In order to determine their ages and petrogenesis, we carried out zircon U–Pb dating, Hf isotope and whole-rock geochemical analyses for four granitic plutons, namely Taohuashan, Dayunshan-Mufushan, Wangxiang and Lianyunshan. Our SIMS zircon U–Pb ages, together with previously published data, reveal that the magmatic activities in this area can be roughly subdivided into three phases at 151–146Ma, 132–127Ma and ca. 117Ma, and the Dayunshan-Mufushan batholith therein is a composite pluton. These four plutons are mainly composed of weakly to strongly peraluminous biotite or two-mica monzogranites, with a minor amount of biotite granodiorites. Their geochemical features are similar to S-type as well as fractionated S-type granites, with enrichment in LREEs and negative Ba, Sr, Nb, P and Ti anomalies. All samples show negative zircon εHf(t) values ranging from −12.5 to −3.6, corresponding to crustal Hf model (TDMC) ages of 1.4–2.0Ga. It is inferred that these granitoids were derived from partial melting of metasedimentary rocks analogous to the Neoproterozoic Lengjiaxi Group, predominantly with psammitic component. Fractional crystallization probably played an important role in the magma evolution, while input of mantle-derived magma was insignificant. Combined with other geological evidence, our new data allow us to propose that the Cretaceous (132–127Ma and ca. 117Ma) magmatism might be response to episodic slab rollback of the Paleo-Pacific plate, while the early-stage (151–146Ma) magmatism that overlapped the epilogue of Jurassic magmatic flare-up and subsequent magmatic quiescence probably foreshadowed the transformation from foundering of a subducted flat-slab to slab rollback. Alternatively, slab foundering after a SE-directed intracontinental subduction in the central SCB cannot be ruled out for geodynamic interpretation of the Jurassic magmatism in SE China.

Fluid-controlled grain boundary migration and switch in slip systems in a high strain, high temperature contact aureole, California, USA

Within the highly strained aureole surrounding the Eureka Valley–Joshua Flat–Beer Creek (EJB) composite pluton of eastern California, an inversion in microstructures and crystallographic preferred orientations (CPOs) exists with distance from the contact. An inner aureole (<250m from the contact) consists of quartzites that are interbedded with marbles and calc-silicates. These quartzites are incompletely recrystallized. Most grain boundaries have migrated, although it is clear that grain boundary migration (GBM) is not extensive. Multiple data sets indicate that temperatures of deformation were above 650°C. CPOs are indicative of <a> slip in quartz. Within the outer aureole (250m to 1500m from the contact), quartzites are interbedded with pelitic schist and are completely recrystallized and microstructures are indicative of extensive GBM. CPOs are indicative of prism [c] slip. Oxygen isotope ratios in the inner aureole are only slightly shifted from their original values. Oxygen isotopes from the outer aureole are shifted more, which is consistent with equilibration with locally derived fluids. We suggest that recrystallization in the outer aureole was aided by pore water, water derived from fluid inclusions, and water generated by prograde reactions in the schists. The pore fluids in the inner aureole were also probably initially water-rich. However, during prograde reactions in the intervening calc-silicate rocks, and perhaps more importantly, between calcite cement and quartz in the quartzites, the pore fluid composition in the inner aureole changed to become dominated by CO2, which acted as a non-wetting phase and decreased the fugacity of water slowing grain boundary mobility. Low water fugacity also suppressed the activity of prism [c] slip. Therefore, we propose that dry conditions or a grain boundary fluid with a significant non-wetting component (CO2) can result in apparent temperatures of deformation that are more than 100°C lower than the real temperatures of deformation.

新丰猴岩岩体LA-ICP-MS锆石U-Pb年龄及地质意义

猴岩岩体位于佛冈复式岩体北东缘，总体呈南北向展布，出露面积约30 km2，岩性多为中细粒黑云母二长花岗岩、花岗闪长岩。本文对猴岩岩体花岗闪长岩样品进行LA-ICP-MS锆石U-Pb测年，获得的同位素年龄为(155.5 ± 4.2) Ma (MSWD = 2.0)，表明猴岩岩体侵位时代为晚侏罗世中期，是燕山第三幕岩浆活动之产物。通过对比，猴岩岩体与佛冈主体花岗岩的侵位时代相一致, 表明二者形成于同期构造–岩浆活动，猴岩岩体是佛冈主体花岗岩分异结晶之产物。猴岩岩体的花岗闪长岩是岩浆分异结晶时捕掳地层残留体，导致岩石成分产生差异而形成，为岩体的边缘相。佛冈复式岩体形成于古太平洋板块向欧亚大陆板块俯冲的构造背景下，其主体花岗岩的地球化学特征表明其物质主要来源于地壳，属陆壳改造型(即S型)花岗岩。 Located at the northeast of Fogang composite pluton, Houyan pluton, which is mainly composed of intermediate-fine grained biotite monzogranites and granodiorites, generally presents a north- south directional distribution and covers an area of 30 km2. In this paper, the LA-ICP-MS zircon U-Pb age of granodiorites of the Houyan pluton is (155.5 ± 4.2) Ma (MSWD = 2.0). The result shows that the emplacement age of Houyan pluton belonged to the middle stage of Late Jurassic, and it was the product of the third magma activity of Yanshan. As its age is the same as the main granite of Fogang composite pluton, they were in the same period of tectono-magmatic activity, and Houyan pluton was crystallized from magmas of main granite of Fogang composite pluton. As the crystallization differentiation of magmas captured the remained stratums, it caused differences in rock composition, and formed the marginal facies which are composed of granodiorites. Fogang composite pluton was formed in the process of subduction of the Pacific plate under Eurasian plate. The geochemical characteristics of its main granite indicate that its source mainly derived from the crust, which belongs to crustal transformation type (S-type) granite.

Thermal history of the giant Qulong Cu–Mo deposit, Gangdese metallogenic belt, Tibet: Constraints on magmatic–hydrothermal evolution and exhumation

A complete thermal history for the Qulong porphyry Cu–Mo deposit, Tibet is presented. Zircon U–Pb geochronology indicates that the mineralization at Qulong resulted from brecciation-veining events associated with the emplacement of a series of intermediate-felsic intrusions. Combined with previously published ages, our results reveal a whole intrusive history of the Qulong composite pluton. Causative porphyries were emplaced at ~16.0Ma as revealed by 40Ar–39Ar dating of hydrothermal biotite (15.7±0.2Ma) and sericite (15.7±0.2Ma). Zircon and apatite (U–Th)/He (ZHe and AHe) dating of Qulong revealed that both followed similar, monotonic thermal trajectories from 900°C (U–Pb ages: 17.5–15.9Ma) to 200°C (ZHe: 15.7–14.0Ma), and that the causative porphyries experienced faster cooling at a maximum rate of greater than 200°C/myr. The Qulong deposit was exhumed between 13.6Ma and 12.4Ma (AHe) at an estimated rate of 0.16–0.24mm/y, which is consistent with previous estimates for other Gangdese Miocene porphyry deposits. Our AHe thermochronology results suggest that neither the Gangdese thrust system, nor the Yadong–Gulu graben affected or accelerated exhumation at the Qulong deposit.