Multiple Tectonothermal Events in the Granulite Blocks of Southern India Revealed from EPMA Dating: Implications on the History of Supercontinents
Multiple Tectonothermal Events in the Granulite Blocks of Southern India Revealed from EPMA Dating: Implications on the History of Supercontinents
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DOI：
10.1016/s1342-937x(05)70643-2
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发表时间：
2003
影响因子：
6.1
通讯作者：
M. Santosh;K. Yokoyama;S. Bijusekhar;J. Rogers
中科院分区：
文献类型：
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作者：
M. Santosh;K. Yokoyama;S. Bijusekhar;J. Rogers
我们使用电子探针技术报告了印度南部四个主要麻粒岩块的29个样品的锆石，独居石，铀矿和hutton的年龄数据。本研究分析的岩石涵盖了这些地区所有主要的岩石单元，包括含石榴石和不含石榴石的charnockites，石榴石-黑云母片麻岩，孔雀岩，钙硅酸盐岩石和一套正长石（黑云母片麻岩，黑云母-角闪片麻岩）。分析了两种代表岩浆期的粉红色变长岩。马德拉斯地块锆石的等时线在2.5-2.6 Ga之间清晰可见。单个锆石颗粒的岩心-边缘分析显示，年龄分带为2.6 ~ 2.9 Ga的火成岩岩心被2.4 ~ 2.5 Ga的岩缘包裹。碎屑锆石年龄可达约3.2 Ga。该块独居石具有2.5 ~ 2.3 Ga的芯和边缘。来自马杜赖地块的19个样品揭示了该地形的多个构造-热事件。正长石的等时线分布清晰，分别为1.7±0.1 Ga、0.82±0.05 Ga和0.58±0.04 Ga。其他岩石的锆石颗粒保留了0.8 ~ 1.0 Ga的岩心或次生生长年龄。该块体的粉红色偏长花岗岩锆石等时线清晰，核心为0.68±0.03 Ga，次级部分为0.57±0.01 Ga。在整个块体中观察到晚泛非叠印，锆石边缘、独居石、铀矿和赫脱石的屈服年龄在0.45-0.60 Ga之间。Trivandrum和Nagercoil地块的锆石均显示0.55 Ga为主要构造热事件，0.8 ~ 1.0 Ga和1.7 ~ 2.0 Ga的构造热事件迹象微弱。Trivandrum块体和Nagercoil块体的独居石数据与马杜赖块体的数据基本相似，只是前者存在残余的独居石。我们的研究证实了Palghat-Cauvery剪切带标志着北部太古宙克拉通和南部元古宙地形之间的主要地形边界。这也加强了该剪切带以南古元古代增生和泛非洲地块改造的观点。印度南部地形的产生、增生和改造年龄为哥伦比亚（~ 1.8-1.5 Ga）、罗迪尼亚（~ 1.1-1.0 Ga）和冈瓦纳（~ 0.6-0.5 Ga）超大陆的历史提供了重要信息。南印度地形形成了以1.8 Ga为中心的全球造山带网络的一部分，勾勒出哥伦比亚的轮廓。此时，在Palghat-Cauvery剪切带以北的Madurai地块上的地形向太古代岩石的增生，与印度东部的东高止山带和东南极洲沿海的Rayner带最古老的变质事件的年龄一致。我们的数据证实了早期的证据，即在泛非造山运动时期（~ 0.5 Ga）冈瓦纳最后一次增生期间，印度最南部的所有地区都经历了同位素系统的重置。Trivandrum和Nagercoil地形在~ 0.7 Ga之后被吸积到马杜赖地块的可能性表明，这个泛非带也可能涉及海洋盆地的吸积和闭合。如果发生这种情况，那么这个区域可能是人们长期寻找的东、西冈瓦纳之间的缝合线。
We report age data on zircon, monazite, uraninite and huttonite from a suite of 29 samples covering four major granulite blocks in southern India using an electron microprobe technique. The rocks analysed in this study cover all of the major lithounits in these terrains and include garnet-bearing and garnet-free charnockites, garnet-biotite gneisses, khondalites, calc-silicate rocks, and a suite of orthogneisses (biotite gneiss, biotite-hornblende gneiss). Two pink metagranites representing the magmatic phase were also analysed. Zircons from the Madras Block yield well-defined isochrons at 2.5-2.6 Ga. Core to rim analyses of single zircon grains show age zoning with 2.6-2.9 Ga igneous cores mantled by 2.4-2.5 Ga rims. Detrital zircons show age up to ca. 3.2 Ga. Monazites in this block have cores and rims with 2.5-2.3 Ga. A suite of 19 samples from the Madurai Block brings out the multiple tectonothermal events in this terrain. Zircons from an orthogneiss yield well-defined isochrons at 1.7±0.1 Ga, 0.82±0.05 Ga and 0.58±0.04 Ga from core, inner rim and rim portions, respectively. Zircon grains in other rocks preserve either core or secondary growth ages at 0.8-1.0 Ga. Zircons in a pink metagranite from this block show sharply defined isochrons of 0.68±0.03 Ga for the core and 0.57±0.01 Ga for the secondary portion. A late Pan-African overprint is observed throughout this block with zircon rims, monazite, uraninite and huttonite yielding age values in the range of 0.45-0.60 Ga. Zircons from both the Trivandrum and Nagercoil blocks show a major tectonothermal event at 0.55 Ga with faint indications of previous tectonothermal events during 0.8-1.0 and 1.7-2.0 Ga. Monazite data from both the Trivandrum and Nagercoil blocks are essentially similar to those from the Madurai Block except for presence of relic monazite in the former. Our study confirms the notion that the Palghat-Cauvery Shear Zone marks the major terrain boundary between an Archean craton in the north and Proterozoic terrains in the south. It also strengthens the view of Paleoproterozoic accretion and Pan-African reworking of blocks south of this shear zone. The ages of production, accretion, and reworking in the terrains of southern India yield important information for the histories of Columbia (∼1.8-1.5 Ga), Rodinia (∼1.1-1.0 Ga), and Gondwana (∼0.6-0.5 Ga) supercontinents. The southern Indian terrains formed part of a worldwide network of orogenic belts that is centered around 1.8 Ga and outlines the configuration of Columbia. Accretion of terrains in the Madurai block to Archean rocks north of the Palghat-Cauvery Shear Zone at this time is consistent with the ages suggested to be the oldest metamorphic event in the Eastern Ghats orogen of eastern India and the Rayner belt of coastal East Antarctica. Our data confirm earlier evidence that all of southernmost India underwent resetting of isotopic systems during the final accretion of Gondwana at the time of the Pan-African orogeny (∼0.5 Ga). The possibility that the Trivandrum and Nagercoil terrains accreted to the Madurai block after ∼0.7 Ga suggests that this Pan-African zone may also have involved accretion and closure of ocean basins. If that happened, then this zone may be the long-sought suture between East and West Gondwana.