A strontium and neodymium isotopic investigation of the Laramie anorthosites, Wyoming, USA: Implications for magma chamber processes and the evolution of magma conduits in Proterozoic anorthosites
A strontium and neodymium isotopic investigation of the Laramie anorthosites, Wyoming, USA: Implications for magma chamber processes and the evolution of magma conduits in Proterozoic anorthosites
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DOI：
10.1016/0016-7037(95)00368-1
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发表时间：
1996
影响因子：
5
通讯作者：
J. Scoates;C. Frost
中科院分区：
文献类型：
--
作者：
J. Scoates;C. Frost
1.43 Ga Laramie斜长杂岩（LAC）富斜长石堆积岩的锶、钕同位素研究为元古代斜长岩结晶过程中岩浆房和下伏岩浆管道系统的演化提供了新的认识。725 km2LAC以3-4千巴的压力侵入北部怀俄明州太古代岩石与南部夏延带元古代岛弧地体之间的边界。坡山（Poe Mountain）、楚格水（Chugwater）和雪溪（Snow Creek） 3个大型复合斜长岩侵入体和晚滑石侵入体的初始锶和ε n2异位比值分别为0.7034 ~ 0.7055和+0.9 ~−4.9。其同位素比值的变化主要受以下因素的影响：(1)母源幔源高铝辉长岩岩浆的同位素组成；(2)太古宙岩石在地壳上升过程中的污染量；(3)侵位末层岩浆房中补充岩浆和不同同位素组成的常驻岩浆的混合作用。在200 km坡山斜长岩的5 ~ 7 km厚层状斜长岩剖面上，锶同位素和规范的An变化表明，在元古代斜长岩中，即使是相对较小的侵入也需要岩浆的多次输入。最早、地层最低的侵入体是一种富含斜长石的岩浆，它结晶了大量成分均匀的斜长岩。坡山斜长岩在地层较高水平的ISrand或规范An的突变表明，至少有三次岩浆补充的全室期，然后是混合。高铝辉石巨晶与层状堆积之间的锶、钕同位素不平衡与元古代斜长岩中富铝辉石巨晶的高压成因一致。巨晶在相对未受污染的玄武岩母岩浆（高铝辉长岩）的10-12千巴压力下在岩浆室中结晶，并随着逐渐受污染的斜长岩岩浆通过地壳输送。在斜长岩中观察到的同位素组成范围与LAC的高铝辉长岩岩脉几乎相同，支持高铝辉长岩是斜长岩的母体的主张（Mitchell et al., 1995）。同位素数据表明，斜长岩母岩浆在上升过程中受到太古宙正长岩和/或变质长岩的污染。随着侵入年龄的相对减小（Snow Creek→Poe Mountain/Chugwater→troctoites）， ε nd的减小和增大表明岩浆通道随着时间的推移逐渐与地壳污染隔绝。该研究表明，锶和钕同位素系统可用于区分发生在下地壳/上地幔压力下、岩浆底喷井上升期间和元古代斜长岩中单个岩浆房内的过程，并强调在解决富含斜长岩堆积的起源时需要地层控制。
A strontium and neodymium isotopic investigation of plagioclase-rich cumulate rocks in the 1.43 Ga Laramie anorthosite complex (LAC) provides new insights into the evolution of magma chambers and underlying magma plumbing systems during the crystallization of Proterozoic anorthosites. The 725 km2LAC intruded the boundary between Archean rocks of the Wyoming Province in the north and Proterozoic island arc terranes in the south, the Cheyenne belt, at pressures of 3–4 kilobars. Initial strontium and εNDisotopic ratios from the three large composite anorthositic intrusions (Poe Mountain, Chugwater, and Snow Creek) and late troctolitic intrusions range from 0.7034–0.7055 and +0.9 to −4.9, respectively. The isotopic ratios in the cumulates vary as a result of (1) the isotopic composition of the parental mantle-derived high-Al gabbroic magmas, (2) the amount of contamination by Archean rocks during ascent through the crust, and (3) mixing of replenishing magmas and resident magmas with different isotopic compositions in the magma chamber at the final level of emplacement. Strontium isotopic and normative An variations across the 5–7 km thick stratigraphic section of layered plagioclase-rich cumulates in the 200 km2Poe Mountain anorthosite indicate that multiple inputs of magma are needed to construct even relatively small intrusions in Proterozoic anorthosites. The earliest and stratigraphically lowest intrusion was emplaced as a plagioclase-rich magma that crystallized large volumes of compositionally homogeneous anorthosite. At least three subsequent chamber-wide episodes of magma replenishment, followed by mixing, are indicated by abrupt shifts in ISrand/or normative An in the stratigraphically higher levels of the Poe Mountain anorthosite. Strontium and neodymium isotopic disequilibrium between a high-Al clinopyroxene megacryst and the layered cumulates is consistent with a high-pressure origin for Al-rich pyroxene megacrysts in Proterozoic anorthosites. The megacryst crystallized in a magma chamber at pressures of 10–12 kilobars from relatively uncontaminated basaltic parent magma (high-Al gabbro) and was transported through the crust in progressively contaminated anorthositic magma. The range of observed isotopic compositions in the anorthositic rocks is nearly identical to that of high-Al gabbroic dikes in the LAC, supporting the proposition that high-Al gabbros are parental to anorthosite (Mitchell et al., 1995). The isotopic data require that the anorthositic parental magmas were contaminated during ascent through the crust by Archean orthogneisses and/or metapelitic rocks. Decreasing ISrand increasing εNdwith a relative decrease in age of the intrusions (Snow Creek → Poe Mountain/Chugwater → troctolites) indicates that the magma conduit became increasingly insulated from crustal contamination over time. This study indicates that the strontium and neodymium isotopic systems can be used to distinguish between processes that occurred at lower crustal/upper mantle pressures, during the ascent of magma diapirs, and within individual magma chambers in Proterozoic anorthosites, and underscores the need for stratigraphic control when addressing the origin of plagioclase-rich cumulates.