An exotic Mesoarchean microcontinent: The Coorg Block, southern India
An exotic Mesoarchean microcontinent: The Coorg Block, southern India
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DOI：
10.1016/j.gr.2013.10.005
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发表时间：
2015
影响因子：
6.1
通讯作者：
M. Santosh;Qiong-Yan Yang;E. Shaji;T. Tsunogae;M. Mohan;M. Satyanarayanan
中科院分区：
文献类型：
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作者：
M. Santosh;Qiong-Yan Yang;E. Shaji;T. Tsunogae;M. Mohan;M. Satyanarayanan
印度南部的库尔格地块夹在北部的达尔瓦尔岩块和南部的新太古代-元古代地壳块体之间，主要由一套弧岩浆岩组成，包括紫苏花岗岩、与TTG（英云闪长岩-奥长花岗岩-花岗闪长岩）有关的花岗岩类套和长英质火山凝灰岩，以及沿地块周边沿着的少量增生洋壳遗迹。在该地块中，具有岩浆混合作用的同时代镁铁质和长英质岩浆活动在弧形背景下得到了很好的代表。本文介绍了该地块主要岩性的岩石学、地球化学、锆石U-Pb年代学和Lu-Hf同位素组成。根据矿物化学数据计算的变质P-T条件表明，麻粒岩相P-T条件一致，为820-870 °C，压力高达6 kbar。我们的地球化学数据的主要，微量元素和稀土元素的代表性样品的主要岩石类型从Cooorg块证实了弧相关的签名，岩浆生成在收敛的边缘设置。正长花岗岩、黑云母花岗岩、奥长花岗岩、紫苏花岗岩的207 Pb/206 Pb年龄分别为3153.4 ± 9 ~ 3184.0 ± 5.5Ma、3170.3 ± 6.8Ma、3275 ± 5.1Ma、3133 ± 12 ~ 3163.8 ± 6.9Ma，镁铁质包体为3156 ± 10 ~ 3158.3 ± 8.2Ma，闪长岩为3161 ± 16 Ma，长英质火山凝灰岩为3173 ± 16 Ma。紫苏花岗岩锆石上交点年龄为3363 ± 59 Ma，下交点年龄为2896 ± 130 Ma，并分别用207 Pb/206 Pb年龄估算了锆石各区域的Th/U值，表明中太古代岩浆侵位时间可能在> 3.3 ~ 3.1 Ga之间，随后在约3.5 ~ 3.5 Ga之间发生了变质作用。3.0到2.9 Ga。紫苏花岗岩及其基性麻粒岩包体、火山凝灰岩和黑云母花岗岩中的岩浆锆石年龄都非常一致和一致，标志着ca。3.1 Ga是该地块俯冲相关地壳建造的高峰期，处于活动会聚边缘的构造环境中。大部分的锆石从Coorg岩石显示Hf同位素特征的典型结晶岩浆来自年轻的来源。他们的Hf地壳模式年龄表明，地壳建设也可能涉及部分再循环的基底岩石，如约。3.8佐治亚州印度南部麻粒岩地体中的地壳块体保留了2.5Ga、2.0Ga、0.8Ga和0.55Ga时期与各种俯冲-增生-碰撞或裂谷事件相关的主要构造事件的强烈印记。然而，Coorg区块是例外，我们的数据表明上述事件均未影响该区块。重要的是，也没有记录在Coorg块的2.5 Ga普遍的区域变质作用，影响了该地区的所有其他块。地质年代学数据提出了一个有趣的可能性，即该地块是印度南部麻粒岩地体北方域中占主导地位的新太古代拼贴中的一个外来实体。中太古代的弧相关的岩石在库尔格块表明，在早期地球的岩浆工厂和他们的构造架构与现代风格的板块构造没有显着不同。
Sandwiched between the Dharwar Craton in the north and the Neoarchean–Proterozoic crustal blocks to the south, the Coorg Block in southern India is composed dominantly of a suite of arc magmatic rocks including charnockites, TTG (tonalite–trondhjemite–granodiorite)-related granitoid suite and felsic volcanic tuffs together with minor accreted oceanic remnants along the periphery of the block. Coeval mafic and felsic magmatism with magma mixing and mingling in an arc setting is well represented in the block. Here we present the petrology, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes of all the major lithologies from this block. Computation of metamorphic P–T conditions from mineral chemical data shows consistent granulite-facies P–T conditions of 820–870 °C and up to 6 kbar. Our geochemical data from major, trace and REE on representative samples of the dominant rock types from the Coorg Block corroborate an arc-related signature, with magma generation in a convergent margin setting. The zircon data yield weighted mean207Pb/206Pb ages of 3153.4 ± 9 to 3184.0 ± 5.5 Ma for syenogranites, 3170.3 ± 6.8 Ma for biotite granite, 3275 ± 5.1 Ma for trondhjemite, 3133 ± 12 to 3163.8 ± 6.9 Ma for charnockites, 3156 ± 10 to 3158.3 ± 8.2 for mafic enclaves, 3161 ± 16 Ma for diorite and 3173 ± 16 Ma for felsic volcanic tuff. An upper intercept age of 3363 ± 59 Ma and a lower intercept age of 2896 ± 130 Ma on zircons from a charnockite, as well as an evaluation of the Th/U values of the zircon domains against respective207Pb/206Pb ages suggest that the Mesoarchean magma emplacement which probably ranged from > 3.3 to 3.1 Ga was immediately followed by metamorphism at ca. 3.0 to 2.9 Ga. The ages of magmatic zircons from the charnockites and their mafic granulite enclaves, as well as those from the volcanic tuff and biotite granite, are all remarkably consistent and concordant marking ca. 3.1 Ga as the peak of subduction-related crust building in this block, within the tectonic milieu of an active convergent margin. The majority of zircons from the Coorg rocks show Hf isotope features typical of crystallization from magmas derived from juvenile sources. Their Hf crustal model ages suggest that the crust building might have also involved partial recycling of basement rocks as old as ca. 3.8 Ga. The crustal blocks in the Southern Granulite Terrane in India preserve strong imprints of major tectonothermal events at 2.5 Ga, 2.0 Ga, 0.8 Ga and 0.55 Ga associated with various subduction–accretion–collision or rifting events. However, the Coorg Block is exceptional with our data suggesting that none of the above events affected this block. Importantly, there is also no record in the Coorg Block for the 2.5 Ga pervasive regional metamorphism that affected all the other blocks in this region. The geochronological data raise the intriguing possibility that this block is an exotic entity within the dominantly Neoarchean collage in the northern domain of the Southern Granulite Terrane of India. The Mesoarchean arc-related rocks in the Coorg Block suggest that the magma factories and their tectonic architecture in the Early Earth were not markedly different from those associated with the modern-style plate tectonics.