来自 Alag-Khadny 高压复合体(蒙古西南部)的闪长岩的矿物学、地球化学和钕-锶同位素特征:大陆内部断裂是大陆边缘俯冲的前兆

IF 1 4区 地球科学 Q3 GEOCHEMISTRY & GEOPHYSICS
S. Yu. Skuzovatov, M. A. Gornova, A. A. Karimov
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引用次数: 0

摘要

在俯冲-增生复合体中,高压岩(蓝闪长岩、斜长岩)通常与低品位岩并置,后者代表其逆退的对应岩,或在后期卷入增生事件,因此是增生带演化的不同阶段的特征。在蒙古西南部,中亚造山带包括新元古代-早古生代的古俯冲复合体,其代表是闪长岩和阿拉格-哈德尼增生复合体的伴生岩。本文报告了对这一复合体中的闪长岩进行矿物学、地球化学和同位素研究的结果。所研究岩石的质地各不相同,有细粒和中粒粒状和线粒状闪石-斜长石-橄榄石岩石,也有中粒状线粒状闪石-橄榄石-赤铁矿-钛铁矿闪长岩,这些岩石在晚期变形过程中经历了强烈的再结晶。原生组合包括辉石和镁角闪石([B]Na = 0.07-0.16, IVAl = 0.79-1.69, [A](Na + K + 2Ca) = 0.14-0.64, [C](Al+ Ti + Fe3+) = 0.58-1.29, Fe2+/(Fe2+ + Mg) = 0.18-0.46 at Fe3+/(Fe3++Al) = 0.18-0.77.77)、低到中等Ca的斜长石(An24-36)和闪石-闪长岩(0.08 < ({{X}_{text{F}}{{text{e}}}^{{text{3 + }}}}}}}}) < 0.16),而逆行集合体则以白云石和镁角闪石为代表。利用闪石成分和闪石/闪石-斜长石热压仪进行的计算显示,峰值P-T条件可达570-630°С和7-9千巴,归因于高T的闪石-斜长石面,随后出现绿辉石派逆变。闪长岩的主要元素组成与低碱中钛透辉岩一致,但其痕量元素组成从N-MORB到E-MORB型玄武岩差异很大,LREE、Nb、Ta、Th、U元素不同程度富集,Eu和Ti元素呈现负异常,这是由于前驱岩的母熔体分馏造成的。钕(εNd(550)从+5.1到-9.1)和锶((87Sr/86Sr)550 = 0.7057-0.7097)的同位素组成表明,岩浆岩的地幔源主要具有明显的中度贫化性质,但也突出了具有非辐射钕组成的 "异常 "地幔域的参与。这些数据支持了闪长岩的前身岩石是在大陆边缘的大陆内延伸过程中形成的,这很可能与一个有限的新元古代大洋盆地的开辟以及随后的晚文武世-早寒武世的汇聚有关。闪长岩的中高压变质作用与斜长岩及相关基岩的逆行变质作用具有相似的P-T条件,与早古生代的俯冲-充填变质作用(约550-540Ma)直接相关,或者是湖泊带与Dzabkhan地层之间的构造混杂带(约515-490Ma或更年轻)形成过程中最终增生的结果。
本文章由计算机程序翻译,如有差异,请以英文原文为准。

Mineralogical, Geochemical, and Nd-Sr Isotope Characteristics of Amphibolites from the Alag-Khadny High-Pressure Complex (SW Mongolia): Intracontinental Rifting as a Precursor of Continental-Margin Subduction

Mineralogical, Geochemical, and Nd-Sr Isotope Characteristics of Amphibolites from the Alag-Khadny High-Pressure Complex (SW Mongolia): Intracontinental Rifting as a Precursor of Continental-Margin Subduction

Within subduction-accretion complexes, high-pressure rocks (blueschists, eclogites) are commonly juxtaposed with lower-grade rocks, which represent their retrograded counterparts or were involved into accretionary event at later stages, and thus characterize distinct stages of evolution of accretionary belts. In SW Mongolia, the Central Asian Orogenic Belt includes Neoproterozoic–Early Paleozoic paleosubduction complexes represented by eclogites and associated rocks of the Alag-Khadny accretionary complex. This paper reports the results of mineralogical, geochemical and isotopic studies of amphibolites from this complex, the geochemical nature and relationships of which with eclogites have been yet uncertain. The texture of the studied rocks varies from fine- and medium-grained granoblastic and nematoblastic amphibole–plagioclase–epidote rocks to medium-grained nematoblastic amphibole–epidote–albite–titanite amphibolites, which experienced intense recrystallization as a response to late deformations. Primary assemblages include pargasite and Mg-hornblende ([B]Na = 0.07–0.16, IVAl = 0.79–1.69, [A](Na + K + 2Ca) = 0.14–0.64, [C](Al+ Ti + Fe3+) = 0.58–1.29, Fe2+/(Fe2+ + Mg) = 0.18–0.46 at Fe3+/(Fe3++Al) = 0.18–0.77), low-to-medium-Ca plagioclase (An24–36), and epidote–clinozoisite (0.08 < \({{X}_{{{\text{F}}{{{\text{e}}}^{{{\text{3 + }}}}}}}}\) < 0.16), whereas the retrograde assemblage is represented by albite and Mg-hornblende. Calculations using amphibole composition and amphibole/amphibole–plagioclase thermobarometry revealed peak P-T conditions up to 570–630°С and 7–9 kbar ascribed to the high-T epidote-amphibolite facies with subsequent greenschist-facies retrogression. The major-element composition of the amphibolites corresponds to low-alkali moderate-Ti tholeiites, although their trace-element composition varies significantly from N-MORB to E-MORB-type basalts, which are variably enriched in LREE, Nb, Ta, Th, U, and show negative Eu and Ti anomalies due to fractionation of parental melts for precursor rocks. Isotopic composition of Nd (εNd(550) from +5.1 to –9.1) and Sr ((87Sr/86Sr)550 = 0.7057–0.7097) indicates distinct mainly moderately-depleted nature of mantle sources for the mafic rocks, but also highlights the involvement of “anomalous” mantle domains with unradiogenic Nd composition. The data supports that the precursor rocks of the amphibolites were formed during intracontinental extension of a continental margin, which was likely linked to opening of a limited Neoproterozoic oceanic basin with a subsequent Late Vendian–Early Cambrian convergence. The medium- to high-pressure metamorphism of amphibolites had similar P-T conditions to that of retrograde metamorphism of eclogites and associated metasediments and was directly related to the Early Paleozoic subduction-accretion metamorphism (~550–540 Ma), or results from the final accretion during the formation of a tectonic mélange zone between the Lake zone and Dzabkhan terrane (~515–490 Ma or younger).

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来源期刊
Petrology
Petrology 地学-地球科学综合
CiteScore
2.40
自引率
20.00%
发文量
27
审稿时长
>12 weeks
期刊介绍: Petrology is a journal of magmatic, metamorphic, and experimental petrology, mineralogy, and geochemistry. The journal offers comprehensive information on all multidisciplinary aspects of theoretical, experimental, and applied petrology. By giving special consideration to studies on the petrography of different regions of the former Soviet Union, Petrology provides readers with a unique opportunity to refine their understanding of the geology of the vast territory of the Eurasian continent. The journal welcomes manuscripts from all countries in the English or Russian language.
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