Baddeleyite-apatite-spinel-phlogopite (basp) rock in achankovil shear zone, south india, as a probable cumulate from melts of carbonatite affinity
Baddeleyite-apatite-spinel-phlogopite (basp) rock in achankovil shear zone, south india, as a probable cumulate from melts of carbonatite affinity
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DOI：
10.1016/j.lithos.2006.01.004
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发表时间：
2006-08
期刊：
影响因子：
3.5
通讯作者：
V. Rajesh;S. Arai
中科院分区：
文献类型：
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作者：
V. Rajesh;S. Arai
本文报道了印度南部晚新元古代Achankovil剪切带（ACSZ）中一种罕见的、迄今尚未确定的斜锆石-磷灰石-尖晶石-金云母（BASP）岩石的岩相学、矿物学和原位微量元素矿物地球化学。该岩石单元作为孤立的露头出现，并划定了ACSZ超镁铁质杂岩的西部边界。玄武岩的模式矿物学为斜锆石<磷灰石<尖晶石<金云母。斜锆石是尖晶石和金云母中的包裹体。尖晶石是高铝质的，含有大量的高温熔体、纯CO2流体、菱镁矿和石墨的夹杂物。磷灰石是F、Cl、LREE、U、Th等卤素元素的主要载体，金云母是Ba、Rb的主要储集体。磷灰石的REE含量变化很大（REE = 7646-13485 ppm），但球粒陨石标准化配分模式显示为均匀的负倾斜REE配分模式，具有显著的负Eu异常，表明磷灰石的结晶环境为还原环境。磷灰石-金云母-斜锆石矿物组合是各种碳酸岩杂岩中碳酸岩的典型矿物组合。磷灰石的轻稀土富集和负Eu异常，金云母的低Ti、Fe和高Al、Mg、Ba、Rb，斜锆石的锕系元素相对于稀土元素的富集，是不同构造环境下碳酸岩的特征。因此，我们认为，BASP岩石可能是由来自上地幔和侵位到下地壳水平的造山带的拉伸崩溃后的碰撞组装的冈瓦纳超大陆的碳酸盐岩亲和力的熔体的晶体沉淀形成的。深根的ACSZ充当了熔体渗透的通道。
We report here the petrography, mineralogy and in-situ trace-element mineral geochemistry of a rare and hitherto unidentified baddeleyite-apatite-spinel-phlogopite (BASP) rock in Late Neoproterozoic Achankovil Shear Zone (ACSZ) in South India. The rock unit occurs as isolated outcrops and demarcates the western boundary of an ultramafic complex in ACSZ. The modal mineralogy of the BASP rock is baddeleyite<apatite<spinel<phlogopite. Baddeleyite is found as inclusions within spinel and phlogopite. The spinel is highly aluminous with copious inclusions of high-T melts, pure CO2-fluids, magnesite and graphite. Apatite is the major carrier of halogens such as F and Cl, LREEs, U and Th, and phlogopite is the robust reservoir of Ba and Rb. The concentrations of REE in apatite are quite variable (ΣREE∼7646–13485 ppm), but the chondrite normalized patterns show a uniform negative sloped REE patterns with significant negative Eu anomaly, which is indicative of the crystallization of apatite in a reduced environment. The mineral assemblage of apatite–phlogopite–baddeleyite is typical of carbonatites from various carbonatite complexes. Moreover, the LREE enrichment and negative Eu anomaly of apatite, low Ti and Fe, and high Al, Mg, Ba and Rb of phlogopite, as well as actinide enrichment of baddeleyite relative to REEs, are characteristic signatures of carbonatites from various tectonic settings. Therefore, we argue that the BASP rock is probably formed by crystal precipitation from melts having carbonatite affinities derived from the upper mantle and emplaced into the lower crustal level during the extensional collapse of the orogen following the collisional assembly of the Gondwana supercontinent. The deep-rooted ACSZ acted as the pathway for the infiltrating melts.