Multistage melt/fluid modification of lithospheric mantle beneath the circum-cratonic orogenic belt: Evidence from the Tuoyun peridotite xenoliths

Xiao Bian, Yuping Su, Jian‐Ping Zheng, Jian Wang, Xi Chen, Liang Zhou, Bi’an Dong, Tianyi Niu
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Abstract

The juvenile and moderate refractory mantle beneath the circum-cratonic orogenic belt is traditionally believed to be hotter and thinner than the ancient refractory cratonic mantle; it is thus more unstable and subject to modification by melts/fluids. Understanding these modification processes would help to elucidate the evolution of Earth’s continents. Peridotite xenoliths carried by the Tuoyun Cenozoic lamprophyre from the southwestern Tianshan belt show evidence of widespread multistage melt/fluid modification of the unstable circum-cratonic orogenic belt mantle. Tuoyun peridotites mainly consist of moderately refractory to fertile lherzolites (Mg# in Ol: 85.5−90.7; Cr# in Sp: 12.7−26.5) and show strong mechanical modification. They can be divided into four groups (A, B, C1, and C2) based on petrography and mineral chemistry. Group A lherzolites show relatively high basaltic components (Al2O3, CaO, TiO2, and FeO) and are enriched in large ion lithophile elements (LILEs) and rare earth elements (REEs), which indicates melt-peridotite reaction processes at high melt/rock ratios. The high modal pyroxene content in Group A suggests that the addition of high-Si melts caused the transition from olivine to pyroxene. Group B lherzolites show high modal pyroxene but relatively depleted incompatible elements, which should be superimposed by later melt extraction. Comparatively, Group C lherzolites exhibit higher modal olivine but lower basaltic components. The clinopyroxene cores of Group C1 are characterized by high (La/Yb)N and low Ti/Eu content, negative high field strength element (HFSE) anomalies, and relatively high 87Sr/86Sr ratios (cores: 0.70331−0.70457), which suggest metasomatism by carbonatite melts originating from recycled sedimentary carbonate. The Group C1 clinopyroxene (spongy rims) and Group C2 clinopyroxene (cores and spongy rims) have positive Sr anomalies, depleted HFSEs, and spoon-shaped REE patterns, which suggest modification by evolved small-volume and volatile-rich silicate melts. In addition, the melt pockets around spinels and the reactive zones of pyroxenes near the lamprophyre reveal the recent incongruent dissolution induced by the host rock. Based on our research and previously reported geological data, we propose that the high-Si melts and carbonatite melts are the products of dehydration and partial melting of the Paleo-Asian oceanic crust, and lithospheric delamination and fracturing (e.g., the Talas-Fergana strike-slip fault) provided the opportunity for small-volume and volatile-rich silicate melts and basaltic melts to modify the peridotites. Multistage melts/fluids and the deformation process are the protagonists in the evolutionary process of the circum-cratonic lithospheric mantle, with important implications for mantle destabilization and multilayered interaction.
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环克拉通造山带下岩石圈地幔的多级熔融/流体改造:来自托云橄榄岩奇石的证据
传统上认为,环克拉通造山带下的幼年和中度难熔地幔比古老的难熔克拉通地幔更热、更薄;因此更不稳定,易受熔体/流体的改变。了解这些改造过程有助于阐明地球大陆的演化过程。天山西南带托云新生代灯盏花岩所携带的橄榄岩异长岩表明,不稳定的环克拉通造山带地幔受到了广泛的多级熔融/流体改造。托云橄榄岩主要由中度耐火至肥沃的蛭石组成(Ol中的Mg#:85.5-90.7;Sp中的Cr#:12.7-26.5),并表现出强烈的力学改造。根据岩相学和矿物化学可将它们分为四组(A、B、C1 和 C2)。A组蛭石显示出相对较高的玄武岩成分(Al2O3、CaO、TiO2和FeO),并富含大离子亲岩元素(LILEs)和稀土元素(REEs),这表明在高熔融/岩石比率下的熔融-橄榄岩反应过程。A 组的高模态辉石含量表明,高硅熔体的加入导致了橄榄石向辉石的转变。B 组叶腊石显示出较高的模量辉石含量,但不相容元素相对较少,这应该是后来熔体萃取叠加的结果。相对而言,C 组 lherzolites 显示出较高的橄榄石模量,但玄武岩成分较低。C1 组鳞片辉石岩心的特点是(La/Yb)N 含量高,Ti/Eu 含量低,高场强元素(HFSE)为负异常,87Sr/86Sr 比值相对较高(岩心:0.70331-0.70457),这表明碳酸盐岩熔体的变质作用源于再循环沉积碳酸盐。C1组clinopyxene(海绵状边缘)和C2组clinopyxene(岩心和海绵状边缘)具有正的Sr异常、贫化的HFSE和勺状的REE模式,这表明它们是由进化的小体积和富挥发性硅酸盐熔体改造而成的。此外,尖晶石周围的熔体袋和灯泡岩附近的辉石反应带也显示了主岩最近引起的不协调溶解。根据我们的研究和以前报告的地质数据,我们提出高硅熔体和碳酸盐岩熔体是古亚洲洋壳脱水和部分熔化的产物,岩石圈的分层和断裂(如塔拉斯-费尔干纳走向滑动断层)为小体积和富含挥发性的硅酸盐熔体和玄武岩熔体改造橄榄岩提供了机会。多级熔体/流体和变形过程是环克拉通岩石圈地幔演化过程的主角,对地幔失稳和多层相互作用具有重要影响。
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