The Shallow Magmatic Plumbing System of the Deccan Traps, Evidence from Plagioclase Megacrysts and Their Host Lavas
The Shallow Magmatic Plumbing System of the Deccan Traps, Evidence from Plagioclase Megacrysts and Their Host Lavas
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德干地盾的浅层岩浆管道系统，来自斜长石巨晶体及其宿主熔岩的证据
DOI：
10.1093/petrology/egac075
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发表时间：
2022
影响因子：
3.9
通讯作者：
Costa, Fidel
中科院分区：
文献类型：
--
作者：
Marzoli, Andrea;Renne, Paul R;Andreasen, Rasmus;Spiess, Richard;Chiaradia, Massimo;Ruth, Dawn C;Tholt, Andrew J;Pande, Kanchan;Costa, Fidel
本文利用来自德干大火成岩省的巨大斜长石玄武岩（GPB）熔岩流的岩石和矿物数据，研究了德干大火成岩省的浅层管道系统，但重点研究了索拉什特拉半岛、马尔瓦高原和西高止山脉（WG）熔岩堆的底部和顶部。WG中的GPB熔岩通常发生在化学性质不同的玄武岩地层之间的过渡阶段。GPB样品多为演化玄武岩，铁、钛含量高，主微量元素和Sr-Nd-Pb同位素组成与德干玄武岩基本相似。主元素模拟表明，GPB玄武岩典型的高铁、演化的熔体可能是由演化程度较低的德干玄武岩在一个普遍干燥的岩浆管道系统中通过低压分馏结晶而形成的。玄武岩为强斑岩，有6-25%的毫米至厘米大小的斜长石巨晶，常以结块的形式出现，还有相对罕见的橄榄石和斜辉石。斜长石晶体多为拉布拉长石，但也有部分斜长石芯（钙长石mol%一般范围：78 ~ 55）。一个共同的特征是在斜长石边缘有很强的铁富集，表明与富铁熔体的相互作用类似于基体成分（FeOt高达16-17 wt%）。利用激光烧蚀电感耦合等离子体质谱分析斜长石微量元素、微量元素和Sr同位素组成，显示出岩浆混合成因的证据。特别是，一些斜长石晶体显示出可变的87sr /86Sri，仅与周围基体的87sr /86Sri部分重叠。扩散模型表明，大多数斜长石巨晶的停留时间为数十年至数百年。值得注意的是，一些斜长石晶体块显示变形的结构证据，通过电子背散射衍射分析和化学图记录，这表明斜长石巨晶是在熔体存在的富晶体环境中变形的。我们将斜长石巨晶解释为最初形成于德干玄武岩浅管道系统的晶体糊状物的残余物。在这种环境下，斜长石由于化学性质不同的玄武岩岩浆的到来而获得了分带组成。在火山爆发之前，迅速上升的富含铁元素的岩浆破坏了浅层的晶体，使其中的一部分重新活动起来，并携带了大量的浮力斜长石巨晶。我们的研究结果表明，来自德干圈闭的玄武岩岩浆，可能来自一般的LIPs，是在复杂的跨地壳岩浆管道系统中产生的，在浅层地壳中发育了广泛的晶体糊状物。
We investigate the shallow plumbing system of the Deccan Traps Large Igneous Province using rock and mineral data from Giant Plagioclase Basalt (GPB) lava flows from around the entire province, but with a focus on the Saurashtra Peninsula, the Malwa Plateau, and the base and top of the Western Ghats (WG) lava pile. GPB lavas in the WG typically occur at the transition between chemically distinct basalt formations. Most GPB samples are evolved basalts, with high Fe and Ti contents, and show major and trace elements and Sr-Nd-Pb isotopic compositions generally similar to those of previously studied Deccan basalts. Major element modeling suggests that high-Fe, evolved melts typical of GPB basalts may derive from less evolved Deccan basalts by low-pressure fractional crystallization in a generally dry magmatic plumbing system. The basalts are strongly porphyritic, with 6–25% of mm- to cm-sized plagioclase megacrysts, frequently occurring as crystal clots, plus relatively rare olivine and clinopyroxene. The plagioclase crystals are mostly labradoritic, but some show bytownitic cores (general range of anorthite mol%: 78–55). A common feature is a strong Fe enrichment at the plagioclase rims, indicating interaction with an Fe-rich melt similar to that represented by the matrix compositions (FeOt up to 16–17 wt%). Plagioclase minor and trace elements and Sr isotopic compositions analyzed by laser ablation inductively coupled plasma mass spectrometry show evidence of a hybrid and magma mixing origin. In particular, several plagioclase crystals show variable87Sr/86Sri, which only partially overlaps with the87Sr/86Sriof the surrounding matrix. Diffusion modeling suggests residence times of decades to centuries for most plagioclase megacrysts. Notably, some plagioclase crystal clots show textural evidence of deformation as recorded by electron back-scatter diffraction analyses and chemical maps, which suggest that the plagioclase megacrysts were deformed in a crystal-rich environment in the presence of melt. We interpret the plagioclase megacrysts as remnants of a crystal mush originally formed in the shallow plumbing system of the Deccan basalts. In this environment, plagioclase acquired a zoned composition due to the arrival of chemically distinct basaltic magmas. Prior to eruption, a rapidly rising but dense Fe-rich magma was capable of disrupting the shallow level crystal mush, remobilizing part of it and carrying a cargo of buoyant plagioclase megacrysts. Our findings suggest that basaltic magmas from the Deccan Traps, and possibly from LIPs in general, are produced within complex transcrustal magmatic plumbing systems with widespread crystal mushes developed in the shallow crust.