A Stable Isotope Study of Anorogenic Magmatism in East Central Asia
A Stable Isotope Study of Anorogenic Magmatism in East Central Asia
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DOI：
10.1093/petrology/37.5.1063
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发表时间：
1996-10
影响因子：
3.9
通讯作者：
S. Wickham;A. Alberts;A. Zanvilevich;B. Litvinovsky;I. Bindeman;E. Schauble
中科院分区：
文献类型：
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作者：
S. Wickham;A. Alberts;A. Zanvilevich;B. Litvinovsky;I. Bindeman;E. Schauble
本文对外贝加尔地区168个深成火成岩和5个水样进行了稳定同位素研究，包括318个全岩和矿物的~（18）O/~（16）O分析和14个D/H分析。这是对该地区巨大的古生代富钾（主要是非造山）花岗岩带的首次详细同位素研究，这些花岗岩带长达数千公里，年龄范围超过250 Ma。在五个主要的侵入岩套中，最年轻的侵入岩套（二叠纪-三叠纪）的侵位伴随着强烈的大气热液活动，局部导致18 O极度亏损（δ 18 O长石<−12）。这反映了这些系统中水的δ 18 O非常低，可能δ 18 O < − 20，δD < − 150，与中生代早期外贝加尔湖的高古纬度（北纬180 °）相一致。尽管局部侵位后，热液18 O亏损的影响，在二叠纪-三叠纪和年轻的岩体，岩浆同位素组成的变化在空间和时间上的五个侵入套，可以清楚地辨别使用18 O/16 O的变化相，如石英和辉石，是抵抗亚固相线交换。本文介绍了一种用激光衍射技术分析β-烯烃中18 O/16 O的方法，该方法可以分析小至0.4mg的样品（包括花岗岩类β-烯烃单晶），为估算深成火成岩的岩浆δ 18 O值提供了一种有效的方法。外贝加尔5个主要侵入岩套（时代从中古生代到中生代）中，大多数辉石和石英的δ 18 O值在1.0-2.0%‰之间变化，且在单个岩体内和同一套岩体内相隔数十公里或数百公里的岩体之间是一致的。但各岩套的δ 18 O/16 O值都有其独特的变化范围，表明在区域尺度上，这些花岗岩类的岩浆δ 18 O值从最早期的δ 18 O +10到最年轻期的δ 18 O +6，以1.1% ‰的递减率递减。这种进展伴随着K和Zr等元素浓度的增加，以及Sr和Ba等元素浓度的减少。这些系统学需要大规模的深部地壳熔融和混合过程，以产生在如此广阔的区域内的单个岩体和岩体群的成分均匀性，以及地壳与碱性幔源岩浆的渐进混合，以产生δ 18 O值逐渐降低的正长岩和花岗岩。这个过程可能是非造山花岗岩成因和陆内克拉通化过程的一个标志，也代表了一种重要的（尽管很大程度上是神秘的）地壳生长机制。
A stable isotope study of 168 plutonic igneous rock and five water samples from Transbaikalia, East Asia, has been made, including 318 whole-rock and mineral18O/16O analyses and 14 D/H analyses. This represents the first detailed isotopic study of the enormous Phanerozoic K-rich (mostly anorogenic) granitoid belts of this region, which are thousands of kilometers in length and span an age range of >250Ma. Of the five main intrusive suites, emplacement of the youngest (Permo-Triassic) was accompanied by intense meteoric-hydrothermal activity, locally causing extreme18O depletio (δ18Ofeldspar<−12). This reflects the very low δ18O of the water involved in these systems, which probably had δ18O < − 20 and δD < − 150, consistent with the high paleolatitude of Transbaikalia in the early Mesozoic (∼80°N). Despite local post-emplacement, hydrothermal18O-depletion effects near Permo-Triassic and younger plutons, the variation of magmatic isotopic composition in the five intrusive suites, in space and time, can be clearly discerned using the18O/16O variation in phases such as quartz and sphene that are resistant to sub-solidus exchange. A procedure for analyzing18O/16O in sphene using a laser fluorination technique is described: analysis of samples as small as 0.4 mg (including single crystals of sphene from granitoids) is possible and provides an effective way to estimate the magmatic δ18O value of plutonic igneous rocks. Most sphene and quartz δ18O values vary by 1.0–2.0%‰ within each of the five main intrusive suites in Transbaikalia (ranging in age from mid-Paleozoic to Mesozoic), and are uniform both within individual plutons and among plutons of the same suite separated by tens or hundreds of kilometers. However, each suite has a unique range in18O/16O, indicating that, on a regional scale, the magmatic δ18O values of these granitoids decreased progressively in ∼1%‰ decrements from ∼+10 in the earliest group to ∼+6 in the youngest. This progression was accompanied by increases in the concentration of elements such as K and Zr, and decreases in the concentration of elements such as Sr and Ba. These systematics require large scale deep crustal melting and mixing processes to generate the compositional uniformity of individual plutons and groups over such wide areas, and also a progressive hybridization of the crust with alkalic, mantle-derived magmas to generate syenites and granites with progressively lower δ18O values. This process may be a hallmark of anorogenic granitoid petrogenesis and the intracontinental cratonization process in general, and also represents an important (though largely cryptic) crustal growth mechanism.