CO2-rich fluid inclusions in staurolite and associated minerals in a high-pressure ultrahigh-temperature granulite from the Gondwana suture in southern India
CO2-rich fluid inclusions in staurolite and associated minerals in a high-pressure ultrahigh-temperature granulite from the Gondwana suture in southern India
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DOI：
10.1016/j.lithos.2007.07.004
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发表时间：
2008-03
期刊：
影响因子：
3.5
通讯作者：
H. Ohyama;T. Tsunogae;M. Santosh
中科院分区：
文献类型：
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作者：
H. Ohyama;T. Tsunogae;M. Santosh
我们报告了第一个详细的流体包裹体研究镁铝岩石冈瓦纳缝合带沿着帕尔加特-考维利剪切带系统在印度南部。这些岩石在新元古代晚期经历了高压（P>12 kbar）和超高温（T ≥ 990 °C）变质作用，与冈瓦纳超大陆的碰撞组装有关。流体包裹体的结构原生和假次生类发生在斜长石，十字石，蓝晶石在这些岩石。在石榴石和磷灰石中发现了次生包裹体。所有被捕获的流体的熔化温度都在−58.6至−56.6 °C之间，表明主要是纯CO2组成。次生包裹体的均一化温度（Th）较高，为+12.9至+30.0 °C，表明密度较低，为0.59-0.83 g/cm 3。虽然大多数原生和假次生包裹体也含有中等至低密度流体（0.67-0.95 g/cm 3; Th=−3.5至+27.4 °C），但在基质斜长石和变斑十字石中发现了罕见的高密度包裹体（1.00-1.06 g/cm 3; Th=−26.1至−14.1 °C）。高密度原生和假次生包裹体的CO2等容线与岩石顺时针P-T轨迹相交的位置约为8.3-9.0 kbar（1000 °C），与该地区的峰期变质条件大致吻合。由于这些流体包裹体显示在主机高级矿物的生长过程中的圈闭纹理的证据，我们解释的高密度CO2流体的痕迹富含CO2的同变质流体，是在超高温变质作用的峰值。尽管十字石和斜长石中保存了高密度的CO2包裹体，但大多数其他矿物中的原生和假次生包裹体显示出广泛的密度变化，这表明这些包裹体中的流体在岩石从高压到峰值超高温阶段沿着顺时针P-T轨迹减压期间经历了显著的密度反转（密度降低）。这些镁铝麻粒岩中纯CO2包裹体的发现与许多其他麻粒岩相地体的类似观察结果一致，这些麻粒岩相地体中的碳流体可能来自地幔储集层，并通过深部剪切带转移，这是冈瓦纳缝合线的特征。
We report the first detailed fluid inclusion study on Mg–Al rocks from the Gondwana suture zone along the Palghat–Cauvery Shear Zone System in southern India. These rocks underwent high-pressure (P>12 kbar) and ultrahigh-temperature (T∼990 °C) metamorphism during the late Neoproterozoic associated with the collisional assembly of the Gondwana supercontinent. Fluid inclusions of texturally primary and pseudosecondary categories occur in plagioclase, staurolite, and kyanite in these rocks. Inclusions of secondary nature are found in garnet and apatite. All the trapped fluids show melting temperatures in the range of −58.6 to −56.6 °C, suggesting a dominantly pure CO2composition. The secondary inclusions show high homogenization temperatures (Th) of +12.9 to +30.0 °C, indicating low density of 0.59–0.83 g/cm3. Although most of the primary and pseudosecondary inclusions also contain moderate to low-density fluids (0.67–0.95 g/cm3; Th=−3.5 to +27.4 °C), rare high-density inclusions (1.00–1.06 g/cm3; Th=−26.1 to −14.1 °C) were discovered in matrix plagioclase and porphyroblastic staurolite. The estimated CO2isochores for the high-density primary and pseudosecondary inclusions intersect the clockwise P–T trajectory of the rocks at around 8.3–9.0 kbar at 1000 °C, which broadly coincide with the peak metamorphic conditions of the area. Since these fluid inclusions show textural evidence for entrapment during the growth of the host high-grade minerals, we interpret the high-density CO2fluids to be traces of a CO2-rich synmetamorphic fluid that was present during the peak ultrahigh-temperature metamorphism. Despite the preservation of high-density CO2inclusions in staurolite and plagioclase, the primary and pseudosecondary inclusions in most of the other minerals show wide density variation which suggests the fluids in these inclusions underwent significant density reversal (density decrease) during decompression of the rocks from high-pressure through the peak ultrahigh-temperature stage along a clockwise P–T trajectory. The finding of pure CO2inclusions in these Mg–Al granulites conforms with similar observations from many other granulite-facies terrains with the carbonic fluids probably derived from mantle reservoirs and transferred through deep-seated shear zones that characterize this Gondwana suture.