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Petrogenesis of Late Cretaceous Jiangla＇angzong I-type granite in Central Lhasa Terrane, Tibet, China： Constraints from Whole-Rock Geochemistry, Zircon U-Pb Geochronology, and Sr-Nd-Pb-Hf Isotopes

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Acta Geologica Sinica(English Edition) 2018年第4期92卷 1396-1414页

作者： LIU Hong LI Guangming HUANG Hanxiao CAO Huawen YUAN Qian LI Yingxu OUYANG Yuan LAN Shuangshuang LU Menghong YAN Guoqiang Chengdu Center China Geological Survey Chengdu 610081 China Faculty of Earth Resources China University of Geosciences Wuhan 430074 China Sichuan Institute of Metallurgical Geology & Exploration Chengdu 610051 China Tianjin Center China Geological Survey Tianjin 300170 China

The Jiangla＇angzong granite in the northern part of the Central Lhasa Terrane is composed of syenogranite and adamellite. LA-ICP-MS zircon U-Pb analyses suggest that syenogranite has a weighted mean 2±6pb/23SU age of 86±1 Ma （mean square weighted deviation=0.37）, which is in accordance with the muscovite Ar-Ar age （85±1 Ma） of Cu-Au ore-bearing skarns and the zircon U-Pb age （84±1 Ma） of adamellite. This suggests that the Jiangla＇angzong magmatism and Cu-Au mineralization events took place during the Late Cretaceous. The granite contains hornblende, biotite, and pyroxene, and does not contain Al-bearing minerals, such as muscovite, cordierite, and garnet. It has high contents of SiO2 （65.10 -70.91wt%）, K20 （3.44-5.17wt%）, and total K20＋Na20 （7.13-8.15wt%）, and moderate contents of A12Oa （14.14-16.45wt%） and CaO （2.33-4.11wt%）, with a Reitman index （δ43） of 2.18 to 2.33, and A/ CNK values of 0.88 to 1.02. The P205 contents show a negative correlation with SiO2, whereas Pb contents show a positive correlation with SiO2. Th and Y contents are relatively low and show a negative correlation with the Rb contents. These characteristics suggest that the Jiangla＇angzong granite is a high K calc-alkaline metaluminous I-type granite. It is enriched in light rare earth elements （LREE） and large ion lithofile elements （LILE）, and depleted in heavy rare earth elements （HREE） and high field strength elements （HFSE）, with LREE/HREE ratios of 11.7 to 18.1. The granite has negative Eu anomalies of 0.58 to 0.94 without obvious Ce anomalies （δCe=l.00-1.04）. The relatively low initial a7Sr/a6Sr ratios of 0.7106 to 0.7179, positive ε±nt（t） values of 1.0 to 4.1, and two-stage Hf model ages （TDM2） ranging from 889 Ma to 1082 Ma, These geochemical features indicate that the granite derived from a juvenile crust. The （143Nd/144Nd）t values from the Jiangla＇angzong granite range from 0.5121 to 0.5123, its eNd（t） values range from -10.17 to -6.10, its （^206pb /^204pb）t values range from 18.683

关键词： Bangong Co-Nujiang Jiangla'angzong 1-type granite Zircon U-Pb Sr-Nd-Pb-Hf

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Two types of granites in the Western Yangtze Block and Their Implications for Regional Tectonic Evolution： Constraints from Geochemistry and Isotopic Data

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Acta Geologica Sinica(English Edition) 2018年第1期92卷 89-105页

作者： MABI Awei YANG Zhengxi ZHANG Mingchun WEN Dengkui LI Yanlong LIU Xuyang School of Earth Sciences Chengdu University of Technology Chengdu 610059 Sichuan China Panxi Team Sichuan Bureau of Geology and Mineral Resources Xiehang 615000 Sichuan China Department of Geological Sciences The University of Texas at E1 Paso E1 Paso 79968 Texas USA

In the western Yangtze Block, widespread Mesoproterozoic to Neoproterozoic rocks are the key to understanding the Precambrian tectonic-magmatic evolution of the region. However, their petrogenesis and tectonic setting are still controversial. In this paper, zircon U-Pb ages, Sm-Nd isotopic and whole-rock geochemical data are reported from selected fresh samples in the southern Dechang county, southwestern China, in order to constrain their emplacement age and magma source, as well as their petrogenesis and tectonic setting. They are mainly composed of biotite monzogranite, monzonitic granite, biotite granodiorites, and quartz diorite. Two ages of 1055 ± 43 Ma and 837.6 ± 3.8 Ma were obtained through zircon U-Pb dating by LA-ICP-MS and LA-MC-ICP-MS, respectively. According to their major element compositions, the Grenville-age granites are peraluminous calc-alkaline series calcic S-type granite. In contrast, the mid-Neoproterozoic granites are metaluminous calc-aikaline series alkalic I-type granite. Furthermore, the S-type granites are enriched in LREEs relative to HREEs with （La/Yb）N ratios of 3.85-18.56 and underwent major fractionation with strongly negative Eu anomalies （Eu/Eu＊ = 0.38-0.66）. In the MORB-normalized trace element variation diagram, all the samples are enriched in Ce and large ion lithophile elements such as Rb, Th, and K, and depleted in high field strength elements such as Nb, and Ti, with negative Sr and Ti anomalies. The I-type granites are enriched in LREEs with slight negative Eu anomalies （Eu/Eu＊= 0.83-0.93）. They are characterized by the enrichment of highly incompatible elements （such as K, Rb, Ba, Th） and LREEs, relative to MORB. Neodymium isotopic data show that the S-type granites display 143Nd/144Nd values of 0.51241-0.51256, and have eNa （t = 1055 Ma） values of （-3.29） to （-3.81）. Calculated tDM ages yield values from 1.87 to 1.91 Ga with the tDM.2stg ages of 1.86 to 1.9 Ga. The I-type granites have 143Nd/144Nd ratios between 0.51192

关键词： 1-type granite S-type granite geochemistry petrogenesis tectonic setting Western YangtzeBlock

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