Subduction polarity of the prototethys: insights from the Yirba pluton of the western Kunlun range, NW China

Abstract

The Yirba (128 km) pluton is an early Paleozoic dioritic intrusion of western Kunlun orogenic belt, northwest China as an important element when reconstructing the evolution history of this belt. Due to the scarcity of field data and methodological difference in studying this pluton, however, no consensus for its age, source and tectonic setting has been adopted. In this paper, we present new geochronological and geochemical data for the Yirba pluton, aiming to better understand its age, source, and hence the early Paleozoic tectonic evolutionary history of western Kunlun. U-Pb data by single grain zircon analyses suggest that the Yirba pluton was emplaced 471 ± 5 Ma ago and contains ca. 490 Ma zircon grains inherited from source, or captured in the magma chamber. The pluton is enriched in Al 2 O 3 (15.7% ∼ 18.4%), Sr (470 ∼ 864 μg/g) and other LILEs (large ion lithosphile elements), but relatively depleted in HFSE (high field strength elements and HREE), with LREE-enriched patterns and low to medium europium anomalies (δEu = ∼ 0.7), showing typical characteristics of I-type, volcanic arc granitoids. Although its relatively high Al 2 O 3, Sr and low MgO contents make it resemble adakite, its relatively high Yb (1.92 ∼ 2.88 μg/g), Y (19.4 ∼ 34.0 μg/g) contents, low Sr/Y (24.2 ∼ 37.0) , Zr/Sm (7.3 ∼ 21) and relatively high initial Sr isotope ratios (0.7075 ∼ 0.7091) do not support a subducting slab origin. Its Nd model ages (1.06 ∼ 1.35 Ga) indicate a juvenile source, while its O isotope compositions (+5.7‰ ∼ + 7.4‰) and Sr-O isotope relationship preclude significant involvement of sialic materials. The major, trace, REE and Nd-Sr-O isotope compositions strongly suggest that the Yirba pluton was formed by partial melting of mafic lower crust in a southward growing, active continental margin environment. The existence of volcanic arc granitoids in the south margin of the North Kunlun terrane suggests that the subduction polarity of the Proto-Tethys was northward.

128公里巖體是西昆侖造山帶中一個早古生代的花崗閃長巖體,長期以來一直是研究西昆侖構造演化的重要參考依據。然而由于區域地質資料的不足和研究手段的不同,對該巖體的形成年代、源區性質以及構造背景等方面還存在著不同的認識。本文試圖通過地質年代學和地球化學方面的研究,明確128公里巖體的成巖時代和構造背景,進而制約西昆侖的早古生代構造演化。單顆粒鋯石的U-Pb定年結果表明128公里巖體形成于471±5 Ma并含有可能形成于早期巖漿房或繼承自源區的490 Ma左右的鋯石。128公里巖體富Al_2O_3(15.7％～18.4％),Sr(470～864μg/g)和大離子親石元素但相對虧損 高場強元素,相對富集輕稀土且具有低到中等的負銪異常(δEu=～0.7),顯示出典型的Ⅰ型弧花崗巖特征。盡管其富集Al_2O_3、Sr、相對低的MgO含量和Y/Yb比值使其非常類似于埃達克巖,但其相對高的Yb(1.92～2.88μg/g)、Y(19.4～34.0μg/g)含量,低的Sr/Y(24.2～37.0)和Zr/Sm(7.3～21)比值以及相對高的初始Sr同位素組成(0.7075～0.7091)排除了消減板塊在石榴石穩定區發生部分熔融的可能性。低的氧同位素组成( + 5.7％～7.4％) 以及Sr-O 同位素关系表明该岩体并非形成于地慢来源的岩泉与变质围岩间的同化混染。高的稀土含量、明显的稀土分馏以及相对高的Sr 同位素组成表明12 8 公里岩体不大可能形成于受陆源物质影响较小的大洋岛弧, 而更可能形成于活动大陆边缘环境中基性地壳物质的部分熔融。北昆仑地体的南缘存在火山弧型花岗岩的事实表明, 原特提斯的消减方向应当是向北的。