Geochronology and geochemistry of gneissic metagranites in eastern Dabie Mountains: Implications for the Neoproterozoic tectono-magmatism along the northeastern margin of the Yangtze Block
Geochronology and geochemistry of gneissic metagranites in eastern Dabie Mountains: Implications for the Neoproterozoic tectono-magmatism along the northeastern margin of the Yangtze Block
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大别山东部片麻状变花岗岩年代学和地球化学：对扬子地块东北缘新元古代构造岩浆作用的启示
DOI：
10.1007/s11430-010-0035-x
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发表时间：
2010-04
影响因子：
5.7
通讯作者：
Qiu JianSheng
中科院分区：
文献类型：
--
作者：
Hu Jian;Wang XiaoLei;Li Zhen;Xu XiSheng;Qiu JianSheng
大别超高压变质带东缘广泛出露新元古代元岩年龄的片麻岩变质岩。本文选取黄镇、大坝、石马、双河、三足斯5个具有代表性的岩体，对其原岩性质进行了详细的年代学和地球化学研究，以期揭示其对扬子地块东北缘新元古代构造-岩浆演化的意义。这些片麻质变质花岗岩主要由石英、钠长石（寡长石）和钾长石组成，还有少量角闪洞和黑云母，以及一些由于UHP变质作用形成的变质矿物，如云母、绿帘石和少量蓝晶石、石榴石。在其中一些岩体中发现了镁铁质碱性矿物（如镁铁石或镁铁石辉石）。锆石LA-ICP-MS U-Pb定年结果表明，片麻质变质岩原岩形成于中新元古代（770 ~ 780 Ma），在早中生代经历超高压变质作用，在215 Ma左右经历高压榴辉岩相重结晶作用。虽然片麻岩型辉长花岗岩sio2含量普遍较高（70.23% ~ 77.23%），且具有金属-弱过铝质特征（ASI=0.90 ~ 1.05），但不同类型岩体的地球化学特征仍存在一定差异。与三足寺岩体相比，黄镇、大坝、石马和双河的变长花岗岩具有较高的K2O+Na2O含量（7.76% ~ 9.45%）、FeOtotal/(FeOtotal+MgO)比值（0.82 ~ 0.96）、hfse和Ga浓度（104×Ga/Al比值平均达3.07）。结合4块岩体普遍含艾格辉石、锆石饱和温度高（816 ~ 918℃）的特征，认为其原岩属于过碱性a型花岗岩。三珠斯岩体主要含角闪洞变质反应生成的黑云母和绿绿岩，化学上富钙贫钾，K2O/Na2O比值为0.42 ~ 0.54，原岩属钙碱性花岗岩类（i型）。大别山东缘片麻质变质岩具有弧状和裂谷状地球化学特征，其原岩可能是在伸展环境下对原有的弧类地壳进行改造而形成的，表明扬子地块东北缘新元古代中期可能处于被动裂陷的初始阶段，而不是由地幔柱上涌引起的主动裂陷环境。
The gneissic metagranites with Neoproterozoic protolith ages are widely exposed along the eastern margin of the Dabie ultrahigh-pressure (UHP) metamorphic belt. In this paper, five representative plutons, including Huangzhen, Daba, Shima, Shuanghe, and Sanzusi, were selected for a detailed chronological and geochemical study aiming to identify the nature of the protoliths and to reveal their implications for the Neoproterozoic tectono-magmatic evolution along the northeastern margin of the Yangtze Block. These gneissic metagranites consist mainly of quartz, albite (oligoclase) and K-feldspar, and minor amphibole and biotite, as well as some metamorphic minerals due to the UHP metamorphism, such as phengite, epidote and minor kyanite and garnet. Mafic alkaline minerals (e.g., aegirine or aegirine-augite) have been observed in some of these plutons. Zircon LA-ICP-MS U-Pb dating results indicate that the protoliths of the gneissic metagranites have been generated in Mid-Neoproterozoic (770–780 Ma), and suffered both ultrahigh-pressure metamorphism in Early Mesozoic and subsequently high-pressure eclogite-facies recrystallization at about 215 Ma. Although the gneissic metagranites generally have high SiO2contents (70.23%–77.23%) and show metaluminous-weakly peraluminous signatures (ASI=0.90–1.05), there are still some geochemical variances between different plutons. Compared with the Sanzusi pluton, the metagranites from the Huangzhen, Daba, Shima, and Shuanghe have high K2O+Na2O contents (7.76%–9.45%), FeOtotal/(FeOtotal+MgO) ratios (0.82–0.96), HFSEs and Ga concentrations with an average 104×Ga/Al ratio up to 3.07. Combined with the features that the four plutons commonly contain aegirine-augite and have high zircon saturation temperatures (816–918°C), it is suggested that their protoliths belong to peralkaline A-type granites. The Sanzusi pluton generally contains biotite and epidote generated from metamorphic reaction of amphibole, and is chemically enriched in calcium and depleted in potassium with K2O/Na2O ratios ranging from 0.42 to 0.54, suggesting their protoliths should be ascribed to calc-alkaline granitoids (I-type). The gneissic metagranites in the eastern margin of Dabie Mountains show both arc- and rift-like geochemical signatures, and their protoliths likely have been generated by reworking of the preexisting arc-related crust under an extensional setting, suggesting that the northeastern margin of the Yangtze Block during the Middle Neoproterozoic likely have been under the initial stage of a passive rifting rather than in an active rifting setting typically induced by the upwelling of a mantle plume.
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