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首页> 外文期刊>Contributions to Mineralogy and Petrology >Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust-mantle mixing and metamorphism in the deep crust
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Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust-mantle mixing and metamorphism in the deep crust

机译：拉萨地块超钾质火山岩中的异岩：深层地壳-地幔混合和变质的直接证据

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摘要

Felsic granulite xenoliths entrained in Miocene (similar to 13 Ma) isotopically evolved, mantle-derived ultrapotassic volcanic (UPV) dykes in southern Tibet are refractory meta-granitoids with garnet and rutile in a near-anhydrous quartzo-feldspathic assemblage. High F-Ti (similar to 4 wt.% TiO2 and similar to 3 wt.% F) phlogopite occurs as small inclusions in garnet, except for one sample where it occurs as flakes in a quartz-plagioclase-rich rock. High Si (similar to 3.45) phengite is found as flakes in another xenolith sample. The refractory mineralogy suggests that the xenoliths underwent high-T and high-P metamorphism (800-850 degrees C, >15 kbar). Zircons show four main age groupings: 1.0-0.5 Ga, 50-45, 35-20, and 16-13 Ma. The oldest group is similar to common inherited zircons in the Gangdese belt, whereas the 50-45 Ma zircons match the crystallization age and juvenile character (epsilon Hf-i +0.5 to +6.5) of Eocene Gangdese arc magmas. Together these two age groups indicate that a component of the xenolith was sourced from Gangdese arc rocks. The 35-20 Ma Miocene ages are derived from zircons with similar Hf-O isotopic composition as the Eocene Gangdese magmatic zircons. They also have similar steep REE curves, suggesting they grew in the absence of garnet. These zircons mark a period of early Miocene remelting of the Eocene Gangdese arc. By contrast, the youngest zircons (13.0 +/- 4.9 Ma, MSWD = 1.3) are not zoned, have much lower HREE contents than the previous group, and flat HREE patterns. They also have distinctive high Th/U ratios, high zircon delta O-18 (+8.73-8.97 parts per thousand) values, and extremely low epsilon Hf-i (-12.7 to -9.4) values. Such evolved Hf-O isotopic compositions are similar to values of zircons from the UPV lavas that host the xenolith, and the flat REE pattern suggests that the 13 Ma zircons formed in equilibrium with garnet. Garnets from a strongly peraluminous metatonalite xenolith are weakly zoned or unzoned and fall into four groups, three of which are almandine-pyrope solid solutions and have low delta O-18 (+ 6 to 7.5 parts per thousand), intermediate (delta O-18 + 8.5 to 9.0 parts per thousand), and high delta O-18 (+ 11.0 to 12.0 parts per thousand). The fourth is almost pure andradite with delta O-18 10-12 parts per thousand. Both the low and intermediate delta O-18 groups show significant variation in Fe content, whereas the two high delta O-18 groups are compositionally homogeneous. We interpret these features to indicate that the low and intermediate delta O-18 group garnets grew in separate fractionating magmas that were brought together through magma mixing, whereas the high delta O-18 groups formed under high-grade metamorphic conditions accompanied by metasomatic exchange. The garnets record complex, open-system magmatic and metamorphic processes in a single rock. Based on these features, we consider that ultrapotassic magmas interacted with juvenile 35-20 Ma crust after they intruded in the deep crust (>50 km) at similar to 13 Ma to form hybridized Miocene granitoid magmas, leaving a refractory residue. The similar to 13 Ma zircons retain the original, evolved isotopic character of the ultrapotassic magmas, and the garnets record successive stages of the melting and mixing process, along with subsequent high-grade metamorphism followed by low-temperature alteration and brecciation during entrainment and ascent in a late UPV dyke.
机译：西藏南部的中新世（约13 Ma）同位素演化的地幔来源的超钾质火山岩（UPV）岩带中的长英质粒状异种岩是具有石榴石和金红石的难熔偏类类质质，呈近无水石英长石组合。高F-Ti（近似于4 wt。％TiO2和近似于3 wt。％F）金云母以石榴石中的少量夹杂物存在，除了一种样品，其以薄片状存在于富含石英斜长石的岩石中。在另一个异种岩样品中发现了高硅（约3.45）扁晶石为片状。难熔矿物学表明异种岩经历了高T和高P变质作用（800-850摄氏度，> 15 kbar）。锆石显示四个主要年龄组：1.0-0.5 Ga，50-45、35-20和16-13 Ma。最古老的群类似于冈底斯带中常见的遗传锆石，而50-45 Ma的锆石与始新世冈底斯弧岩浆的结晶年龄和幼年特征（εHf-i +0.5至+6.5）相匹配。这两个年龄组一起表明，异种石的成分来自冈底斯弧岩。 35-20 Ma中新世年龄来自锆石，其具有与始新世冈底斯岩浆锆石相似的Hf-O同位素组成。它们也具有相似的陡峭REE曲线，表明它们在没有石榴石的情况下生长。这些锆石标志着始新世Gangdese弧早期中新世重熔的时期。相比之下，最年轻的锆石（13.0 +/- 4.9 Ma，MSWD = 1.3）没有划分区域，其HREE含量比前一组低得多，并且HREE模式平坦。它们还具有独特的高Th / U比，高锆石O-18值（+ 8.73-8.97千分之一）和极低的εHf-i值（-12.7至-9.4）。此类演化的Hf-O同位素组成与来自容纳异种石的UPV熔岩的锆石的值相似，平坦的REE模式表明13 Ma锆石与石榴石形成平衡。来自强钙铝辉石异岩的石榴石被弱带或无带分成四类，其中三类是铝金刚烷-固溶体，低δO-18（千分含量为6至7.5份），中度（δO-18） + 8.5至9.0（千分之几）和高增量O-18（+11.0至12.0千分之几）。第四种几乎是纯净的，辐射度为O-18，千分之10-12。低和中三角洲O-18基团均显示出铁含量的显着变化，而两个高三角洲O-18基团在成分上均一。我们解释这些特征，以表明低和中三角洲O-18石榴石生长在通过岩浆混合而聚集在一起的单独的分离岩浆中，而高三角洲O-18基团则在高级变质条件下伴随着交代交换而形成。石榴石在单个岩石中记录了复杂的，开放系统的岩浆和变质作用过程。基于这些特征，我们认为超钾质岩浆在侵入约15 Ma的深地壳（> 50 km）中时会与35-20 Ma的少年壳发生相互作用，形成杂交的中新世花岗岩体岩浆，并留下耐火渣。类似于13 Ma的锆石保留了超钾盐岩浆的原始，演化同位素特征，石榴石记录了熔融和混合过程的连续阶段，以及随后的高级变质作用，随后在夹带和上升过程中发生了低温变化和破裂。在晚期UPV堤防中。

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