ROLE OF POST-OROGENICEXTENSIONAL TECTONICS IN THE SUPERGIANT ANTIMONY MINERALIZATION IN CENTRAL HUNAN PROVINCE, SOUTH CHINA

The Xikuangshan antimony deposit in central Hunan, South China, is the largest antimony deposit ever known in the world. The ore bodies are strictly confined to the footwalls of mafor high-angle normal faults which transect the inclined folds in the flank;away from the fault planes,both the homogenization temperatures of inclusions in gangue minerals and the intensity of antimony mineralization decrease.These characteristics strongly demonstrate that the faults are used as the conduit for the metal-bearing fluid in mineralization.The normal faults,striking the NE to NNE,are interpreted to be generated by the postmgenic extension in the time from Cretaceous to Paleogene.Crustal or lithospheric thinning, directly resulting from regional extension inevitably increase the geothermal gradient, Which is likely to cause large-scale convection of underground water that may leach out and transport valuablemetals such as Sb from source rocks.Focussed discharge along the fault zones contributes to the formation of the supergiant antimony deposit in Xikuangshan under the appropriate sedimentary barrier.

Early Yanshanian post-orogenic granitoids in the Nanling region——Petrological constraints and geodynamic settings

Early Yanshanian magmatic suites predominate absolutely in the Nanling granite belt. They consist mainly of monzogranite and K-feldspar granite. There occur associations of early Yanshanian A-type granitoids (176 Ma-178 Ma) and bimodal volcanic rocks (158 Ma-179 Ma) in southern Jiangxi and southwestern Fujian in the eastern sector of the granite belt and early Yanshanian basalts (177 Ma-178 Ma) in southern Hunan in the central sector of the belt. Both the acid end-member rhyolite in the bimodal volcanic rock association and A-type granitoids in southern Jiangxi have the geochemical characteristics of intraplate granitic rocks and the basic end-member basalt of the association is intraplate tholeiite, while the basaltic rocks in southern Hunan include not only intraplate tholeiite but also intraplate alkali basalt. Therefore the early Yanshanian magmatic suites in the Nanling region are undoubtedly typical post-orogenic rock associations. Post-orogenic suites mark the end of a post-collision or late

Origin and Tectonic Implications of Post-Orogenic Lamprophyres in the Sulu Belt of China

Lamprophyre dykes that crosscut different types of ultrahigh pressure(UHP)metamorphic rocks are widely distributed in the Triassic Sulu UHP orogenic belt.Although abundant studies have been performed on these dykes,their origin and petrogenesis remain topics of controversy.This study presents the results of a detailed field-based study of petrology,whole-rock geochemistry and zircon U-Pb and Lu-Hf isotopes on lamprophyre dykes exposed in the central Sulu UHP zone,aiming at shedding lights on their petrogenesis and providing clues on the geological evolution of eastern continental China during the Cretaceous.The lamprophyres are typically porphyritic,with phenocrysts dominantly composed of amphibole and clinopyroxene set in a lamprophyric matrix.The dykes have moderate Si O2(47.70 wt.%–60.44 wt.%),variably high Mg O(2.58 wt.%–8.28 wt.%)and Fe2 O3 T(4.88 wt.%–9.26 wt.%)contents with high Mg#of 49–66.Geochemically,they have enriched light rare earth element(REE)and flat heavy REE patterns((La/Gd)N=5.14–10.56;(Dy/Yb)N=1.43–1.54)with negligible Eu anomalies(Eu/Eu*=0.83–1.10),and they show enrichment in large ion lithophile elements(e.g.,Ba and K)but depletion in high-field strength elements(e.g.,Nb,Ti and P).In-situ zircon U-Pb geochronology reveals that the lamprophyres have concordant ages of 120–115 Ma,demonstrating that the dykes emplaced in the Early Cretaceous.These zircons have?Hf(t)values ranging from-26.0 to-11.0.Inherited zircons that occur in the dykes are dated to be Neoproterozoic,in line with the protolith ages of their host(i.e.,the UHP rocks).An integration of these data allows us to propose that the lamprophyres were generated during the Cretaceous,by melting of subcontinental lithospheric mantle-derived metasomatite with enriched chemical compositions underneath the North China Craton.The metasomatite was formed mainly by peridotite-fluid/melt reactions,with the fluids/melts mainly liberated from subducted Yangtze continental crust during the Triassic.Regional extension,lithospheric thinning and mantle upwelling caused by rollback of the subducted paleoPacific plate is considered to account for the generation of the lamprophyres as well as the extensive arc-like magmatic rocks in eastern continental China during the Early Cretaceous.

Zircon SHRIMP Dating for the Weiya Pluton, Eastern Tianshan: Its Geological Implications

The timing of the emplacement of the Weiya pluton remains controversial due to the absence of systematic and precise dating. This paper reports zircon SHRIMP U-Pb dating of different lithologic phases in the Weiya pluton, and discusses the genesis and tectonic environment. The ages of gabbro, quartz syenite, diorite porphyrite and fine-grained granite are 236±6 Ma, 246±6 Ma, 233±8 Ma and 237±8 Ma, respectively. All these phases were formed in early-middle Indosinian （Triassic） in a post-orogenic environment. In addition to underplating, intraplating of mantle-derived magmas is also a substantial mechanism for magma generation and vertical accretion of the continental crust. Granitoid rocks are important products of vertical continental accretion as underplating evolves gradually to intraplating. The existence of post-orogenic Indosinian granites shows that the middle Tianshan orogenic belt underwent an important tectonic conversion from the Paleo-Asian ocean subduction-collision system to the Paleo-Tethys ocean regime.

Geochemical constraints on the origin of Early Cretaceous alkaline intrusions and its tectonic implication,Sulu Orogenic Belt,Eastern North China Craton

Post-orogenic alkaline intrusions from the Sulu Orogenic Belt of eastern North China Craton consist of A-type granites.In this study,we report U-Pb zircon ages,geochemical data,Sr-Nd-Pb,and zircon Hf isotopic data for these rocks.The LA-ICP-MS U-Pb zircon analyses yield consistent ages ranging from 127.1±2.4 to 119.5±4.8 Ma for four samples.The alkaline rocks are characterized by high total alkalis(K2 O+Na2 O=8.32-10.11 wt%),light rare-earth element enrichment,and heavy rare-earth element depletion,with a wide range(La/Yb)N values(20-48),moderate negative Eu anomalies(Eu/Eu*=0.50-0.74),enrichment in large-ion lithophile elements(LILEs,i.e.,Rb,Th,U and Pb),and depletion in Ba,Sr and high field strength elements(HFSEs,i.e.,Nb,Ta,and Ti),high(87Sr/86Sr)i ranging from 0.708 to 0.7089,low sNd(t)values from-19.4 to-16.8,(206Pb/204Pb)i=16.751-16.935,(207Pb/204Pb)i=15.381-15.535,(208-Pb/204Pb)i=37.472-37.838,negativeεHf(t)values between-21.3 and-25.7 for the magmatic zircons,and larger TDM2 model ages from 2.5 to 2.8 Ga.These results suggest that the rocks were derived from a common enriched lithospheric mantle source that was metasomatized by foundered lower crustal eclogitic materials before magma generation.Furthermore,the geochemical and isotopic feature implies that the primary magma of these rocks originated through partial melting of ancient lithospheric mantle that was variably hybridized by melts derived from lower crust eclogite.These rocks in this study may have been generated by subsequent fractionation of potassium feldspar,plagioclase,ilmenite,and/or rutile.However,negligible crustal contamination occurred during the diagenesis process.

Characterization of a Post Orogenic A-Type Granite, Gabal El Atawi, Central Eastern Desert, Egypt: Geochemical and Radioactive Perspectives

The alkali feldspar granite of Gabal El Atawi is post orogenic granite originated from subalkaline magma in extensional suite. It is developed within plate tectonic setting and has A2-type character which generated from apparent crustal source. The petrographic, geochemical and radioactive characteristics of El Atawi granite meet and fulfill the requirements of being fertile granite and it can be considered as promising uraniferous granite. Fluid inclusion studies of the altered granite elucidated two different solutions acting on the host granitic pluton. The first is NaCl-CaCl low temperature fluid with a wide range of salinity. The second is high temperature and salinity Fe-Mg-Na chloride solution. Different fractures in the granite acted as good channels for the hydrothermal fluids that leached uranium from its bearing minerals disseminated all over the host granite and redeposited it in the alteration zones.

Tectonic Setting of the Cu-Ni Sulfide-Bearing Mafic-Ultramafic Complexes in Northern Jilin Province, NE China

There are series of Cu-Ni sulfide-bearing mafic-ultramafic intrusions widespread in north JilinProvince,Northeastern China. The intrusions formed in Xing’an-Mongolian Orogenic Belt near to the northeastern margin of North China Craton. The complexes were formed in almost same period according to the zircon U-Pb dating reported recently, which means that the complexes were formed in same tectonic period and belong to one tectonic magmatic event. The rock assemblages are different from the ophiolite type and Yidun type in orogenic belt. The mafic-ultramafic complexes formed in the range from 217 Ma to 232 Ma coeval with A-type granites in the area, which formed bimodal igneous rock assemblage. According to the regional angular unconformities, there were existed the orogenies of Caledonian, Hercynian, Early Indosinian, Late Indosinian and Yanshanian. The Early Indosinian coeval with orogenic I-type granites and sanukitie that suggesting the lithosphere thickening in the extrusion tectonic setting of orogenic processes, however the Late Indosinian coeval with bimodal igneous rock assemblage that suggesting the lithosphere thinning in the extension tectonic setting of post-orogenic processes in the Xing’an-Mongolian Orogenic Belt. Chemical composition of the mafic-ultramafic rocks has the characteristics of high-Mg and low-K tholeiites related with inter-continental post-orogenic tectonic setting. The trace elements indicate their formed in conditions of continental extension belt or initial rift and has the characteristics of revolution from oceanic island arc, volcanic arc of continental margin to continental extended belt. The low initial Sr isotopic ratios and positive εNd(t) values suggest that the initial magma of the complexes come from the parting melting of depleted lithospheric mantle. The depleted ithospheric mantle was new formed supported by zircon Hf isotope in Hongqiling complex. The depleted lithospheric mantle may be caused by the asthenosheric mantle upwelling and underplating in the tectonic setting of extension during the Late Indosinian post-orogenic processes.

Age, Geochemistry, and Tectonic Implications of Dulaerqiao Granite, Inner Mongolia

Dulaerqiao granite is located at the Xinlin-Xiguitu-Toudaoqiao suture zone between the Erguna massif and the Xing＇an massif, northeast of Inner Mongolia. The rocks are mainly composed of K-feldspar, quartz, and plagioclase. Zircon LA-ICP-MS U-Pb data show that this granite was deposited in the Late Carboniferous Period （308.7±2.0 Ma）. The samples are rich in alkali, Fe, and AI and low in Mg, Ca, and P. Chondrite-normalized REEs exhibit right-inclined patterns with significant negative Eu anomalies. Additionally, the granite shows high quantities of trace elements such as Zr, Hf, Th, K, and Rb and decreased quantities of Sr, P, and Ti. The chemical characteristics identified herein and a series of diagrams that distinguish different types of granite show that Dulaerqiao alkali-feldspar granites belong to the aluminous A-type granite group. Meanwhile, the initial magma crystallizes under high- temperature, low-pressure conditions resulting from a tectonic extension setting. The formation of Du- laerqiao aluminous A-type granite is related to the rejuvenation of the ancient Xinlin-Xiguitu- Toudaoqiao suture zone, which was activated by the interaction between the combined Erguna-Xing＇an massif and the Songnen massif in the Late Paleozoic Era. This aluminous A-type granite was deposited about 30 Ma after the collision.

The First Discovery of a Tin Deposit Related to Hercynian A-Type Granites in China

This paper introduces the petrological, mineralogical and geochemical fea. tures of the rocks hosting the Sareshik tin deposit—an independent tin deposit related to Hercynian A-type granites discovered in China in 1987. A comparison with the features of A-type granites at home and abroad indicates that some of their petrological features are more typical than those of famous A-type granites of Nigeria. The geological features of the tin deposit are salient and notably different from those of some famous tin deposits in China, so the deposit may represent China’s deposits associated with anorogenic granites. This tin deposit was formed in the Hercynian Stage. This type of tin deposit was for the first time discovered in China and is also rare in the world. It is a new type of tin deposit in China.