Garnet Granite Research Articles

Metamorphic evolution of Daqingshan supracrustal rocks and garnet granite from the North China Craton: Constraints from phase equilibria modelling, geochemistry, and SHRIMP U–Pb geochronology

Supracrustal rocks and garnet granite within the high-grade metamorphic complex in Daqingshan, at the northern margin of the Palaeoproterozoic Khondalite Belt, North China Craton (NCC), preserve various mineral assemblages that record three distinct stages of their metamorphic evolution. The mineral assemblages representative of prograde metamorphism (M1) comprise plagioclase (Pl) + quartz (Q) ± biotite (Bio) inclusions preserved in garnet porphyroblasts in gneiss and granite members. The metamorphic peak mineral assemblages (M2) are represented by garnet (Gt) + Pl + Bio + Q + sillimanite (Sil) + K-feldspar (Kfs) in each rock’s matrix. Retrograde mineral assemblages (M3) that formed during post-peak, near-isothermal decompression (ITD) occur between relict garnet porphyroblasts and matrix minerals. These M3 assemblages consist of Bio + cordierite (Crd) + Pl + Gt + Q + Sil + Kfs. Phase equilibria modelling shows that the supracrustal rocks record clockwise pressure–temperature (P–T) trajectories from 650 to 750 °C/5.3–8.9 kbar (M1) up to 800–830 °C/9.8–11.2 kbar (M2), to 760–830 °C/4.8–6.2 kbar (M3). Geochemical similarities between the Daqingshan supracrustal rocks (i.e. garnet-biotite gneisses) and garnet granites, such as relative enrichment in K, Rb, and Ba, and depletion in Th, U, Ta, Nb, P, and Ti, imply a petrological affinity. Sensitive high-resolution ion microprobe analysis (SHRIMP) U–Pb dating of zircons reveals that the protolith of the Daqingshan supracrustal rocks formed at ~2.5 Ga and subsequently metamorphosed at ~2.45–2.37 Ga. Anatexis likely occurred at ~2.43–2.40 Ga during the retrograde near-ITD stage. Combined with previous studies, the timing of metamorphism and anatexis in the Daqingshan area may be connected with the regional mafic magmatism.

The Djilouet granite suite (Djanet terrane, eastern Hoggar, Algeria): petrography, mineralogy, geochemistry, and relations with quartz-cassiterite-wolframite vein systems

The Djilouet complex forms a cupola made up of leucocratic granites associated with Sn-W mineralization. It could represent the Hoggar’s easternmost rare metal granite (RMG) comparatively to those of the Taourirt province in the central Hoggar. It is located in the Djanet terrane, 12 km NE of the town of the same name in the far east of the Tuareg shield in Algeria. The Djanet terrane is made of a thick low-grade (greenschist facies) sedimentary sequence which was intruded by several generations of granitic rocks. The subcircular Djilouet body is made of leucocratic granites with progressive mutual transitions. Most of the outcrop is occupied by a porphyritic coarse-grained biotite granite. A muscovite granite is found in the center of the cupola, whereas a garnet (almandine—spessartite) granite forms a discontinuous rim all around it. Black micas from the biotite granite are lithian annite (“protolithionite”). The white micas from the muscovite granite and the garnet granite are classified as Fe-Li muscovite. The muscovite granite and the garnet granite contain accessory minerals as rutile, xenotime, monazite, zircon, and minerals that may be members of the pyrochlore supergroup. All the facies are leucogranite with high SiO2 content and high K2O-Na2O ratio. They are poor in calcium and in mafic components and have very low phosphorus content (P2O5 ~ 0.1%). The peraluminous composition is low to mild (1.08 1.46). The evolved granites of the Djilouet suite are enriched in Th and U, but the tantalum and the niobium are not highly concentrated (Ta + Nb = 10.4–17.1). The total REE content is low (57–84 ppm), and the patterns are typically wing shaped with a strong negative Eu anomaly and a small tetrad effect. The Sn-W mineralization consists two systems of veins: large quartz veins with ferberite (H/F ~ 0.8) and quartz veinlets or stockworks with homogeneous cassiterite and minor wolframite (H/F ~ 48). The iso-content contours of tin and tungsten, as produced from a sampling covering the whole cupola, overlap very little. The differences which were noted throughout the study between the Taourirt granites and those of the Djilouet suite are to be related to the lithological nature of the crust rather than to a difference in the geodynamic environment.

Zircon U-Pb dating and Hf isotopic compositions of intrusions in the Erdaohe silver-lead-zinc deposit, Inner Mongolia and the metallogenic process of the deposit

二道河银铅锌矿为大兴安岭中段最新发现的一处大型矽卡岩型矿床。银铅锌矿的形成与矿区多期岩体密切相关，矿体主要赋存于矽卡岩、大理岩及细粒花岗闪长岩内。本文对矿区主要岩体及石榴子石矽卡岩开展了锆石LA-ICP-MS U-Pb年代学、Hf同位素研究和石榴子石电子探针分析，获得了绿帘石化细粒花岗闪长岩、绿帘石化细粒闪长岩、花岗斑岩的²⁰⁶Pb/²³⁸U加权平均年龄分别为506±4.3Ma、213±2.8Ma和128±1.1Ma。二道河矿区石榴石矽卡岩与绿帘石矽卡岩常呈相互穿插关系，闪锌矿、方铅矿等矿石矿物具有重结晶特征，矽卡岩矿物石榴子石电子探针分析结果显示在矿区至少存在两期不同性质的流体活动，表明二道河银铅锌矿至少经历了两期以上的矿化事件。获得的三类岩体的锆石ε_Hf（t）值分别为0.9~6.9、8.3~15.1和7.9~14.1，对应的二阶段模式年龄（t_DM2）分别为1.6~1.1Ga、0.8~0.3Ga和0.8~0.3Ga，表明这些花岗岩类均来源于新生地壳部分熔融，它们的成因可能分别与额尔古纳地块和兴安地块的拼合、蒙古-鄂霍茨克洋闭合及古太平洋板块俯冲有关。这些构造体系的叠加与演化对二道河银铅锌矿床的形成和富集具有非常重要的作用。;The Erdaohe silver-lead-zinc deposit is a newly discovered large skarn deposit in central Great Xing’an Range, and its formation is closely related to multi-stage intrusions. The ore bodies in the deposit mainly occur in skarns, marble, and fine-grained granodiorites. In this study, we conducted LA-ICP-MS zircon U-Pb dating, Hf isotopic analysis, and the electron microprobe analysis (EPMA) of garnets for the major intrusions and garnet granites in the deposit. Zircon grains from the epidotized fine-grained granodiorites, epidotized fine-grained diorites, and granite porphyry yielded weighted mean ²⁰⁶Pb/²³⁸U ages of 506±4.3Ma, 213±2.8Ma, and 128±1.1Ma, respectively. The garnet skarns and epidote skarns in the Erdaohe deposit are frequently interspersed with each other, and the ore minerals such as sphalerite and galena in the deposit are characterized by recrystallization. Moreover, the EPMA results of garnets in the skarns show there were at least two stages of fluid activities with different natures. All these indicate that the Erdaohe deposit has undergone at least two stages of mineralization events. Zircons from three types of intrusions obtained yielded ε_Hf(t) values of 0.9~6.9, 8.3~15.1, and 7.9~14.1, respectively, with t_DM2 values of 1.6~1.1Ga, 0.8~0.3Ga, and 0.8~0.3Ga, respectively. This indicates that all these granitoids are originated from the partial melting of the juvenile crust, and their formation may be related to the combination of the Erguna and Xing’an blocks, the closure of the Mongol-Okhotsk Ocean, and the subduction of the Paleo-Pacific Plate, respectively. The superimposition and evolution of tectonic systems play a crucial role in the formation and enrichment of the Erdaohe silver-lead-zinc deposit.

The characteristics of autochthonous and hypautochthonous anatectic granite: A case study of garnet granite in Baotou area, northern margin of the North China Craton

在华北克拉通北缘大青山地区，广泛的深熔作用导致新太古代晚期石榴花岗岩发育。石榴花岗岩空间上与新太古代晚期大青山表壳岩（主要为石榴黑云母片麻岩）共生，渐变过渡。宏观上岩性具有不均一性，在包头哈德门沟一条实测地质剖面上可以观察到石榴混合闪长岩、石榴混合石英闪长岩和石榴混合花岗闪长岩等不同岩石类型。岩相学研究表明，石榴花岗岩与源岩矿物组合相近，但含量却大不相同。石榴花岗岩和源岩中的石榴石特征十分相似。石榴石周围存在放射状黑云母，是典型的逆反应结构。研究表明深熔作用可能发生在具有顺时针<i>P-T</i>轨迹的变质作用演化峰期及峰期后近等温降压阶段，主要发生钾长石熔融和黑云母熔融。地球化学特征研究表明石榴花岗岩与石榴黑云母片麻岩具有亲缘关系，均表现出高Al₂O₃、CaO含量，K₂O/Na₂O比值存在较大变化，富集轻稀土和大离子亲石元素，亏损Nb、Ta、P和Ti。但是，石榴花岗岩存在组成变化，Eu富集型和亏损型稀土模式为原地-半原地深熔花岗岩的标志性特征。SHRIMP锆石年代学研究表明深熔作用发生在2.43Ga之前，与区内新太古代晚期水平顺层滑脱变形有关，揭示华北克拉通北缘新太古代晚期构造热事件。

Timing of magmatism and ultrahigh- to high-grade metamorphism in the Kannak Complex, Kon Tum Massif, Vietnam: Magmatic activity and its tectonic implications

Detailed geochronological and petrological investigations of metamorphic and igneous rocks can significantly improve our understanding of the tectonic processes that occur in orogenic belts. The Kon Tum Massif, central Vietnam, includes various types of magmatic and metamorphic rocks, the latter including ultrahigh- to high-temperature units. The Late Permian–Early Triassic metamorphic rocks in the massif correspond to the metamorphic characteristics of a collision between the Indochina and South China blocks in terms of their metamorphic evolution and timescales of deformation. U–Pb geochronological analyses of zircon obtained from two samples of garnet granite in the massif yielded magmatic ages of 239.6 ± 3.9 Ma and 243.5 ± 3.4 Ma. This permitted the reevaluation of previous Th–U–Pb electron microprobe analyses of monazite from the garnet granite and garnet-orthopyroxene gneiss and previously published U–Pb dating. These data reveal the metamorphic and magmatic history of the western part of the massif. The metamorphic rocks show a thermal evolution from 260 Ma to 240 Ma. Magmatic activity started at 260 Ma and ceased at 240 Ma. Considering this in terms of the regional-scale igneous activities in the Kon Tum Massif, the base of the massif, already thickened by the collision, was situated above underplating plume-related magma. Such magmatic activity resulted in high- to ultrahigh-temperature metamorphism and crustal melting during the Late Permian to Early Triassic, between 260 and 240 Ma.

Petrogenesis and tectonic implication of Paleoproterozoic granites and granulites in the Fengzhen area of North China craton

The North China Craton (NCC) is one of the oldest cratons in the world that preserves the records of many important geological events in the early evolution of the Earth. However, the early Precambrian tectonic evolution history of the NCC is still controversial. Here we report Paleoproterozoic leucogranites, garnet granites and granulites in the Fengzhen area that constitutes a junction between the Khondalite Belt and the Trans-North China Orogen (TNCO). The leucogranites have high SiO2 content (75–85wt%), and magmatic zircon U-Pb age of 2199±3 Ma. Their zircons have slightly various εHf(t) values (−0.45 to 1.41) with TDM model ages of 2.5–2.6Ga. The granites show strong LREE enrichment with La/Ybcn ratio of 12–176, and have high initial Nd isotope (εNd(t)=3.5–4.5). Most of them display positive Eu anomaly, excluding fractional crystallization contribution on the high Si feature. The leucogranite was produced by melting of Late Archean felsic TTGs (tonalite–trondhjemite–granodiorite) and addition of juvenile crust. Magmatic zircons yield the garnet granite age of 2002±3Ma. The zircons show various εHf(t) values (−5.8 to 7.1) and TDM model ages (2.1–2.6Ga). Low initial εNd(t) isotope value (−4.2 to −8.3) was determined in the granites. The granulites are high pressure metamorphic rocks with a peak pressure of 12.0±0.8 kbar, and have similar metamorphic age of 1967±10Ma to the garnet granites. This indicates that the Fengzhen area underwent 2-Ga inter-continental orogeny, in which the melting of sedimentary rock formed the garnet granites. Similarly, sodic TTG melting to form K-rich leucogranites also needs high pressure. However, the leucogranite age is obviously older than the peak metamorphic timing of continent–continent collision. This implies that the Fengzhen area may have underwent arc-continental collision at 2.2Ga.

A preliminary study of rare-metal mineralization in the Himalayan leucogranite belts, South Tibet

The Himalayan leucogranite occurs as two extensive (>1000 km) E-W trending belts on the Tibetan Plateau with the unique features. The leucogranite comprised biotite granite, two-mica/muscovite granite, tourmaline granite and garnet granite, which have been identified in previous studies, as well as albite granite and granitic pegmatite that were identified in this investigation. Fifteen leucogranite plutons were studied and 12 were found to contain rare-metal bearing minerals such as beryl (the representative of Be mineralization), columbite-group minerals, tapiolite, pyrochlore-microlite, fergusonite, Nb-Ta rutile (the representative of Nb-Ta mineralization), and cassiterite (the representative of Sn mineralization) mainly based on the field trip, microscope observation and microprobe analysis. The preliminary result shows that the Himalayan leucogranite is commonly related to the rare-metal mineralization and warrants future investigation. Further exploration and intensive research work is important in determining the rare-metal resource potential of this area.

Earliest Paleoproterozoic supracrustal rocks in the North China Craton recognized from the Daqingshan area of the Khondalite Belt: Constraints on craton evolution

The Upper Wulashan “Subgroup” at Daqingshan in the Khondalite Belt of the Western Block of the North China Craton contains an Early Paleoproterozoic (2.5–2.45Ga) supracrustal sequence, incorporating banded iron formations, which we refer to as the Daqingshan Supracrustal Rocks. They contain rounded to elliptical zircon grains that commonly show core–rim or core–mantle–rim structures and precise SHRIMP U–Pb dating of these domains allows the discrimination of a series of tectono-thermal events that straddle the Archean/Proterozoic boundary. Detrital zircon cores with oscillatory zoning have formation ages ranging from 2.55 to 2.50Ga, indicating derivation from Late Neoarchean magmatic rocks. As a result of exhumation and erosion, these became incorporated in earliest Paleoproterozoic sediments that were deposited between 2.50 and 2.45Ga and then underwent high-grade metamorphism at 2.45–2.40Ga. This event variously recrystallized the cores into two main domains: a dark inner domain and a gray outer domain, as imaged in cathodoluminescence. The gray domains commonly show sector zoning and have Th/U ratios of 0.1–0.5, a feature commonly noted in zircons recrystallized under high-grade metamorphic conditions. Metamorphic mantles are generally more homogeneous in structure and have lower Th/U ratios (commonly <0.1) than the recrystallized domains, although they show similar age distributions. However, they are difficult to distinguish when only two domains are present in the zircon and they represent a continuum of recrystallization. The rocks then underwent a second episode of high-grade metamorphism in the Late Paleoproterozoic, as revealed by thin overgrowth rims that are homogeneous or show only weak zoning, have low Th/U ratios (commonly <0.1), and record ages of 1.95–1.90Ga. This is the first time that two tectono-thermal events of Early and Late Paleoproterozoic age have been identified in single rock samples from the Western Block of the North China Craton. The conclusion that unequivocal earliest Paleoproterozoic sediments are present in the North China Craton is also supported by evidence that the Daqingshan Supracrustal Rocks are cut by anatectic garnet granite, previously dated at 2.39Ga. Combined with evidence obtained here and elsewhere in the North China Craton, we conclude that the 2.45–2.40Ga metamorphism was not an extension of the well-established, craton-wide Late Neoarchean tectono-thermal event, but a previously unrecognized episode that followed a short quiescent period that allowed sedimentation of the Daqingshan Supracrustal Rocks.

Zircon ages of metamorphic and magmatic rocks within peridotite-bearing mélanges: Crucial time constraints on early Carboniferous extensional tectonics in the Chinese Tianshan

We dated and geochemically characterized peridotite-bearing mélanges in the Chinese South Tianshan and within the Main Tianshan Shear Zone. The Yushugou-Tonghuashan mélange in the Chinese South Tianshan exposes a tectonic juxtaposition of a diapirically emplaced metaperidotite (predominantly lherzolite) massif with a high-grade metamorphic terrane (ca. 10km long; protolith age≥ca. 445–466Ma). Metamorphic zircons of a mafic granulite (εNd(t)=5.0) yielded a weighted mean 206Pb/238U age of 341±8Ma that we interpret as the time of granulite-facies metamorphism. The youngest zircon rims of an intermediate granulite (εNd(t)=−4.3) have a mean age of 332±13Ma that records a retrogressive metamorphic event. These ages determine the timing (ca. 341–332Ma) of mantle diapirism through continental crust. A dolerite dike (εNd(t)=2.3) emplaced into metaperidote has a crystallization age of 335±5Ma, that, on the basis of geochemistry, we interpret as representing E-MORB-OIB magmatism that accompanied mantle diapirism. An undeformed pink granite (εNd(t)=−3.6) intrudes the mélange matrix and has an emplacement age of 324±5Ma, thus providing an upper time limit for a tectonic movement that led to mélange formation. The older time limit of deformation (ca. 362–352Ma) is constrained by the youngest ages of thermo-tectonically modified zircons in a mylonitized metagabbro and a foliated meta-andesite. Magmatic zircons in the meta-andesite (εNd(t)=−3.3) and a dacite (εNd(t)=−5.6) have eruption ages of 433±4Ma and 435±3Ma that date formation of the mélange matrix. A microgabbro dike (εNd(t)=10.0) cutting metasediments (i.e. mélange matrix) has an emplacement age of 279±3Ma and contains abundant zircon xenocrysts ranging in age from Archean to late Paleozoic. Similarly, a dolerite dike, a meta-andesite, an intermediate granulite, and a metagabbro all contain inherited Precambrian to Paleozoic zircons. We accordingly conclude that the mélange was formed by mantle diapirism in the early Carboniferous and was subsequently affected by an early Permian volumetrically minor mafic episode. Two foliated garnet granites (εNd(t)=−7.0 and −6.9), in direct contact with metaperidotite within the Main Tianshan Shear Zone have emplacement ages of 341±2Ma and 329±4Ma that reflect an early Carboniferous episode of crustal anatexis. They also contain Neoproterozoic zircon xenocrysts. Also within the Main Tianshan Shear Zone, a gabbro (εNd(t)=8.7) has an emplacement age of 297±3Ma and contains abundant Neoproterozoic to late Paleozoic zircon xenocrysts. Based on the regional geological relationships, we associate early Carboniferous mantle diapirism and crustal anatexis with lower crustal delamination-induced asthenospheric upwelling. The numerous Precambrian zircon xenocrysts support a Chinese Tianshan–Tarim connection. The early Permian mafic episode coincided in time with the development of an early Permian unconformity in the Chinese North Tianshan.

Contribution of crustal anatexis to the tectonic evolution of Indian crust beneath southern Tibet

This geochemical, geochronological and structural study of intrusive rocks in the Sakya Dome of southern Tibet has identified two distinct suites of anatectic granites that carry contrasting implications for the tectonic evolution of the India-Asia collision zone. The northern margin of the dome core was intruded by anastomosing, equigranular two-mica garnet granites between 28.1 ± 0.4 Ma and 22.6 ± 0.4 Ma, coeval with top-to-the-south shear. Trace-element and isotopic (Sr-Nd) characteristics indicate an origin from partial melting of a biotite-bearing source in the Indian crust, under conditions of high fluid-phase activity. These granites thus provide evidence for the melt weakening required by some thermo-mechanical models that predict the southwards extrusion of a low-viscosity channel during the Oligocene. Evidence for subsequent shear-sense reversal may document initiation of this process. However, a younger suite of porphyritic two-mica granite plutons, emplaced between 14.5 ± 0.9 Ma and 8.81 ± 0.22 Ma, are derived from anatexis of muscovite-bearing metasediments of the High Himalayan Series under fluid-absent conditions. Ar-Ar cooling ages of 14.4 to 8.0 Ma from the Sakya dome postdate crystallisation of the Oligocene granite suite by ca. 10 Ma, but are coincident with mid-Miocene granite emplacement, suggesting uplift to depths of <10 km by the mid-Miocene. We propose that plate flexural response to Miocene slab steepening is a likely cause of dome uplift, and that this exhumation of mid-crustal rocks triggered decompression melting at 15-9 Ma and emplacement of discrete granite plutons into the upper crust under brittle conditions.

Deformation features of garnet-bearing granites from Huwan, western Dabie Mountains

Foliated garnet-bearing granite, usually associated with high pressure and ultrahigh -pressure (UHP) metamophic rocks, is a particular rock-type extensively exposed in the Mesozoic Dabie-Sulu orogenic belt of China. This study focuses on deformation features and SHRIMP zircon dating of foliated garnet granite in a high-pressure metamorphic unit from Huwan, western Dabie Mountains in order to resolve discrepancies in current versions of its petrogenesis and structural evolution. SHRIMP dating reveals a zircon age of 762 ± 15 Ma (MSWD=1.7) for Huwan granites, representing the Middle to Late Neoproterozoic age of intrusion and crystallization. Field and microstructural studies show that the Huwan granite body underwent multiple-stage deformation. The deformation was manifested by an early stage of rootless folding and imposition of relict foliation (S1); an Indosinian main stage marked by imposition of north-dipping penetrative gneissosity (S2) and development of ductile shear zones under NNE-SSW directed compression; and a final Indosinian stage of southward thrusting of the Huwan high-pressure unit. Shallow level extension prevailed after the Late Triassic, giving rise to south-dipping thrust faults and north-dipping normal faults.

Plutonic rocks of the Median Batholith in southwest Fiordland, New Zealand: Field relations, geochemistry, and correlation

This paper provides a first description of all major plutonic rock units between Resolution Island and Lake Poteriteri in southwest Fiordland. Plutonic rocks, of which c. 95% are granitoids, comprise c. 60% of the basement in southwest Fiordland. Approximately 50% of the plutonic rocks were emplaced between c. 355 and 348 Ma, 5% at c. 164 Ma, 25%between c. 140 and 125 Ma, and 20% between c. 125 and 110 Ma. These episodes of plutonism occurred in response to terrane amalgamation, continental thickening, and subduction along the convergent margin of Gondwana. Correlatives of Devonian plutonic rocks which occur in Nelson are absent from the area described here. A wide variety of plutonic rocks were emplaced at c. 355–348 Ma. These include relatively small plutons of K‐ and Rb‐rich gabbro‐diorite and members of at least three distinct suites of granitoids. Plutons of two‐mica ± garnet granodiorite, granite, and minor tonalite share affinities with the S‐type Ridge Suite and are the most widespread c. 355–348 Ma old granitoids in southern Fiordland. Plutons rich in Ca, Fe and Zr, depleted in K and Na, and containing quartz diorite, tonalite, and minor granodiorite with the unusual assemblage red‐brown biotite, garnet ± hornblende ± clinopyroxene also occur widely in southern Fiordland. These plutons are similar to peraluminous A‐type granitoids, indicating A as well as I and S‐type plutonism occurred in the Western Province at this time. The Newton River and Mt Evans Plutons have no correlatives amongst c. 355–348 Ma granitoids in southern Fiordland, but their chemistry is similar to that of the older Karamea Suite. Three regional‐scale metasedimentary units—locally fos‐siliferous Fanny Bay Group Buller Terrane rocks in southern Fiordland, Edgecumbe and Cameron Group Takaka Terrane rocks in south‐central Fiordland, and undifferentiated Deep Cove Gneiss high‐grade metasedimentary rocks of western Fiordland—are all stitched by c. 355–348 Ma old plutons, indicating they have been in close proximity since at least c. 355–348 Ma. In south‐central Fiordland, c. 355–348 Ma old plutons cut across fabrics defined by upper amphibolite facies mineral assemblages, indicating low pressure/high temperature metamorphism in this area before this time. The c. 164 Ma old leucocratic Lake Mike Granite is a unique pluton in southwest Fiordland with no obvious correlatives. Plutons emplaced between c. 140 and 125 Ma are similar to the Rahu Suite, although isotopic data are required to confirm this correlation. Rahu Suite plutonism may therefore have begun by c. 140 Ma, rather than c. 120 Ma as previously suggested. Plutons emplaced between c. 125 and 110 Ma have high Sr/Y ratios comparable with the Separation Point Suite. They occur in both an outboard location around Lake Poteriteri and an inboard location around the western end of Dusky Sound. The c. 115 Ma two‐mica garnet granites of the Anchor Island Intrusives #2 probably formed by partial melting of adjacent ortho‐ and paragneisses, indicating that upper amphibolite facies metamorphism in western Dusky Sound occurred during the Early Cretaceous. The Dusky Fault does not pass directly out to the coast through outer Dusky Sound as previously mapped. Instead it merges with the major northeast‐striking Lake Fraser Fault at Cascade Cove, which crosses the outer coast near West Cape. The Last Cove Fault is a minor structure which cannot be traced beyond Last Cove rather than a major fault of regional extent as has been previously suggested.

Crustal anatexis and formation of two types of granitic magmas in the Kontum massif, central Vietnam: Implications for magma processes in collision zones

Asia grew in the Late Permian by the collision of a number of micro-continents. Syn- to post-collisional magmatism occurred along the continental collision zones . In this study, we report two types of granitic rocks, garnet granite (Grt granite) and orthopyroxene granite (Opx granite), from the Kontum massif, central Vietnam, which is situated on the continental collision zone between the South China and Indochina cratons. These granitic rocks were formed at ca. 250 Ma when high-temperature (HT) and ultrahigh-temperature (UHT) metamorphism took place in the same zone. Based on the petrological and geochemical features compared with previously reported experimental results, garnet-bearing granite is derived from pelitic gneisses by partial melting, whereas orthopyroxene-bearing granite is produced by the partial melting of garnet-bearing mafic granulites. We inferred that a significantly high-geothermal gradient is required to produce Vietnamese granitic magmatism and related HT to UHT metamorphism. This geotherm may be attributed to upwelling mantle plume beneath the Kontum massif during the Late Permian.

EPMA Th-U-Pb Monazite Dating of Zhongtiao and Lüliang Precambrian Metamorphic Complexes

Zhongtiao and Lüliang Precambrian metamorphic complexes are essential parts of the southern segment of Trans-North China Orogen (TNCO), the North China Craton (NCC). Their metamorphism and related granitic magmatism enable to better understand the nature and the evolution of the TNCO, the NCC, with great scientific significance. EPMA Th-U-Pb monazite dating of two monzogranitic rock samples from Yongji and Henglingguan intrusions in the Zhongtiao Precambrian complex yields two peak ages of 1884.7 Ma-1849.9 Ma, and 1743.5–1738.8 Ma, respectively. The former is interpreted as the main metamorphic ages and the latter as the ages of fluid activation. Five samples from the Lüliang Precambrian complex were dated by EPMA Th-U-Pb monazite dating method. The garnet two-mica schist and the granitoid gneiss recorded three age peaks or range of >1902 Ma, 1883.3 Ma-1865.6 Ma, and 1731.3 Ma, corresponding to early magmatic and anatectic events, peak metamorphism, and late fluid activation, respectively. The two samples of garnet granite yield the ages of 1882.8 Ma-1850.9 Ma, corresponding to the anatectic granitic magmatism, which simultaneously occurred with the peak metamorphism. The unmetamorphosed granite vein yields the age of 1742.6 Ma, which is interpreted as the forming age of the granite vein and together with the age of 1731.3 Ma shown by the garnet two-mica schist may imply important fluid-magmatic activation events. The above dating results reveal that the peak metamorphism occurred during 1885 Ma-1849 Ma in the Zhongtiao and the Lüliang Precambrian metamorphic complexes, accompanied by the S-type granitic magmatism. The age range of the peak metamorphism and cogenetic S-type granitic magmatism in the Zhongtiao and Lüliang Precambrian complexes are consistent with the metamorphic and the deformation events in the Hengshan–Wutai–Fuping complexes, further proving that the amalgamation between the Western and the Eastern continental blocks in the NCC occurred during late Paleoproterozoic.

Timing of metamorphism and formation of garnet granite in the Kontum Massif, central Vietnam: Evidence from monazite EMP dating

Garnet-bearing granite (Grt-granite) and high-grade pelitic gneisses occur in the Kannak Complex, southern Kontum Massif, central Vietnam. Electron microprobe (EMP) dating of monazite in Grt-granite and garnet-orthopyroxene gneiss (Grt-Opx gneiss) gave ages of ca. 260 Ma from cores and ca. 230 Ma from thin rims. Since most monazite grains are predominately of core composition, the ca. 260 Ma age indicates the timing of high-temperature metamorphism and the formation of Grt-granite. Grt-granite is locally accompanied by coeval intrusions of fine-grained gabbro, which is a candidate for the heat source leading to high-grade metamorphism and partial melting of the crust during the Late Permian.

Timing of metamorphism and formation of garnet granite in the Kontum Massif, central Vietnam: Evidence from monazite EMP dating

Garnet-bearing granite (Grt-granite) and high-grade pelitic gneisses occur in the Kannak Complex, southern Kontum Massif, central Vietnam. Electron microprobe (EMP) dating of monazite in Grt-granite and garnet-orthopyroxene gneiss (Grt-Opx gneiss) gave ages of ca. 260 Ma from cores and ca. 230 Ma from thin rims. Since most monazite grains are predominately of core composition, the ca. 260 Ma age indicates the timing of high-temperature metamorphism and the formation of Grt-granite. Grt-granite is locally accompanied by coeval intrusions of fine-grained gabbro, which is a candidate for the heat source leading to high-grade metamorphism and partial melting of the crust during the Late Permian.

Geology of the plutonic basement rocks of Stewart Island, New Zealand

Exposures of basement rocks on Stewart Island provide a c. 70 km long by 50 km wide map of part of the Median Batholith that spans the margin of the Western Province. Because of their distance from the present plate boundary, these rocks are relatively unaffected by Cenozoic tectonism, allowing examination of unmodified Carboniferous‐Cretaceous relationships within the Median Batholith. Thirty individual plutons (>c. 20 km2) have been mapped along with numerous relatively small intrusions (<c. 5 km2). The large plutons form 85–90% of the Median Batholith on Stewart Island while the many smaller intrusions comprise 10–15%, mostly in the north. Lithologies include: biotite ± minor hornblende granodiorite, granite and leucogranite with accessory titanite ‐ magmatic epidote and allanite (c. 50%); biotite ± muscovite ± garnet granite with S‐type affinities (c. 10%); alkaline quartz monzonite, granite, and alkali feldspar granite with rare aegirine and blue‐green amphibole (c. 3%); quartz monzodiorite and diorite with hornblende > biotite (c. 23%); gabbro and anorthosite (c. 12%) and ultramafic rocks (c. 2%). U‐Pb zircon and monazite dating indicates that c. 12% of these plutonic rocks were emplaced during the Carboniferous between 345 and 290 Ma, c. 20% in the Early‐Middle Jurassic at c. 170–165 Ma, c. 30% in the latest Jurassic to earliest Cretaceous between 152 and 128 Ma, and c. 38% in the Early Cretaceous between 128 and 100 Ma. The distribution of Pegasus Group schists and peraluminous granitoid rocks indicates that the northern limit of extensive early Paleozoic Western Province basement is located either within the Gutter Shear Zone or at the Escarpment Fault, 10–15 km south of the Freshwater Fault System previously thought to mark this boundary. Carboniferous and Middle Jurassic magmatism extended plutonic basement northwards as far as the Freshwater Fault System, while further magmatism during the latest Jurassic and earliest Cretaceous produced the basement north of the Freshwater Fault System. The focus of Early Cretaceous plutonism then returned southwards into the Western Province, although the older basement in this area was only involved in the genesis of subordinate peraluminous plutonism at this time and not the more extensive metaluminous rocks. The Escarpment Fault disrupted this c. 40 km wide section across the margin of the Western Province at c. 110–100 Ma.

Two linear granite belts in the central-western North China Craton and their implication for Late Neoarchaean-Palaeoproterozoic continental evolution

Abstract Precambrian granitoid rocks are widely distributed in the North China Craton (NCC). The dominant granitic rocks in the central-western NCC are trondhjemitic, tonalitic and granodioritic (TTG) gneisses, as well as some calc-alkaline granitic gneisses and minor charnockites. Two linear granite belts in this area are different from other early Precambrian granitic rocks in the NCC in terms of their occurrence and chemistry; the Liang-Tu (LT) garnet granite belt and the Sanggan (SG) granite belt. The LT belt occurs along two large-scale ductile shear zones in metamorphosed Al-rich sedimentary country rocks (khondalitic series). In the central part of the LT belt are metagabbroic granulite lenses that were originally mafic dykes. In the outer part of the belt, garnet granites grade into khondalitic rocks. LT belt granites have geochemical characteristics of S-type granites. The SG granite belt is ∼400 km long and composed of many deformed gneissic granitic bodies and granitic sills and veins with/without gneissosity. They contain various lenses including high-pressure granulites and retrograded eclogites. The SG granite belt trends east-northeast, parallel to the regional gneissosity of the country rocks. Major and trace element geochemistry of these granites shows characteristics of syn-collisional and post-collisional granite. The country rocks beside the Sanggan granite belt are different, including the Huai’an-Fengzhen terrain (HFT) to the northwest and the Wutai-Fuping terrain (WFT) to the southeast. The LT and SG granite belts formed within an old continental craton and an old orogenic zone between two blocks, respectively.

Petrology of granitic rocks along new Pos Selim to Kampung Raja highway (km 0 to km 22): identification of different granitic bodies, its field and petrographic characteristics

Four granitic bodies have been identified along the Pos Selim-Kampung Raja Highway (0 to km 18). They are Selim granite (km 0 to km 5), Suku granite (from km 6 to km II), Semerengol aplopegmatite (occur as granitic dykes intruded the metamorphic rocks at km 14 to km 15) and Regul granite (km 17 to km 18). The Selim granite consists of outer coarse grained porphyritic biotite granite and medium to fine grained granite. The Suku granite is characterized by the presence of xenoliths and other enclaves. Among the enclaves are biotite rich, quartz tourmaline pods and various types of xenoliths (metamorphic). The Regul granite at km 17 and 18 contains primary muscovite. The Semerengol aplopegmatite consists of tourmaline muscovite garnet granite and their aplopegmatite dykes found at km 14 and 15. Geochemical evidence showed that three granitic bodies (Selim, Suku and Regul granites) are different in term of their Ti02, Fep3 and pps contents. This supports our field observation and division of the granites along Pos Selim­ Kampung Raja Highway.

Timing of the granulite facies metamorphism in the Sanggan area, North China craton: zircon U-Pb geochronology

Zircon U-Pb ages are reported for three samples of intrusive rocks in Khondalite series in the Sanggan area, North China craton. The age of meta-granite is dated as 2005±9 Ma, implying that the sedimentary sequences in Khondalites series formed before 2.0Ga. The age of 1921±1Ma for the meta-diorite constrain the age of granulite facies metamorphism younger than this date. The age of 1892±10 Ma for garnet granite is obtained, but the granite crystallization age seems a little younger than the date considering the morphology of zircons. On the basis of these dates and of a concise review of previous age data, it is inferred that the Khondalite series was subjected to granulite facies metamorphism at about 1.87Ga together with tonalitic granulites and HP basic granulites in the Sanggan area.