Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust–mantle mixing and metamorphism in the deep crust
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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DOI：
10.1007/s00410-016-1272-6
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发表时间：
2016-06
影响因子：
3.5
通讯作者：
Rui Wang;W. Collins;R. Weinberg;Jinxiang Li;Qiuyun Li;Wen-yan He;J. Richards;Z. Hou;Li-min Zho
中科院分区：
文献类型：
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作者：
Rui Wang;W. Collins;R. Weinberg;Jinxiang Li;Qiuyun Li;Wen-yan He;J. Richards;Z. Hou;Li-min Zho
藏南中新世(~13 Ma)地幔超钾质火山(UPV)岩墙中的长英质麻粒岩包体是一种近无水的石英长石质组合中含有石榴石和金红石的难熔变质花岗岩类。高F-Ti(~4wt.%TiO2和~3wt.%F)金云母以小包裹体的形式赋存于石榴石中，但有一个样品以片状赋存于富含石英斜长石的岩石中。另一个捕虏体样品中发现高硅(~3.45)多硅白云母呈片状分布。难熔矿物学研究表明，包体经历了高温、高磷变质作用(800~850℃，15kbar)。锆石主要有四个年龄组：1.0~0.5Ma，50~45 Ma，35~20 Ma，16~13 Ma。最古老的一组与冈底斯带常见的继承锆石相似，而50-45 Ma的锆石与始新世冈底斯弧岩浆的结晶年龄和年轻特征(εHfi+0.5~+6.5)相匹配。这两个年龄组共同表明，包体的一部分来自冈底斯弧岩。35~20 Ma的中新世年龄来源于锆石，其Hf-O同位素组成与始新世冈底斯岩浆锆石相似。它们也有相似的陡峭的稀土元素曲线，表明它们是在没有石榴石的情况下生长的。这些锆石标志着始新世冈底斯弧早中新世重熔时期。相比之下，最年轻的锆石(13.0 Ma±24.9 Ma，MSWD=91.3)没有分带，HREE含量远低于前一组，且HREE模式平坦。它们还具有独特的高Th/U比值、高的锆石δ18O(+8.73-8.97‰)值和极低的εHfi(−12.7-−9.4)值。这种演化的Hf-O同位素组成与赋存捕虏体的UPV熔岩中的锆石的值相似，平坦的稀土配分模式表明，13 Ma的锆石与石榴石是平衡形成的。强过铝变质英云闪长岩捕虏体中的石榴石分带较弱或未分带，分为4组，其中3组为铁铝榴石-镁铝榴石固溶体，具有低δ18O(+6~7.5n‰)、中等(δ18O+8.5~9.0‰)和高δ18O(+11.0~12.0‰)。第四种几乎是纯的安山岩，δ18O 10-12‰。低δ18O组和中等δ18O组在Fe含量上均有显著差异，而两个高DNA18O组在成分上是均匀的。我们对这些特征的解释表明，低、中δ18O群石榴石生长在通过岩浆混合聚集在一起的分离分馏岩浆中，而高δ18O群形成于伴随交代交换的高级变质条件下。石榴石记录了单一岩石中复杂的、开放系统的岩浆和变质过程。根据这些特征，我们认为，超钾质岩浆在~13 Ma侵入地壳深部(&Gt；50 Ma)，形成杂化的中新世花岗岩类岩浆后，与年轻的35~20 Ma地壳相互作用，留下难熔的残留物。~13 Ma锆石保留了超钾质岩浆原始演化的同位素特征，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，以及在UPV晚期岩墙夹带和上升过程中的低温蚀变和角砾岩作用。这是西藏地壳深部原地壳幔混合的一个很好的例子。
Felsic granulite xenoliths entrained in Miocene (~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 (~4 wt.% TiO2and ~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 (~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 °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 (εHfi+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 δ18O (+8.73–8.97 ‰) values, and extremely low εHfi(−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 meta-tonalite xenolith are weakly zoned or unzoned and fall into four groups, three of which are almandine-pyrope solid solutions and have low δ18O (+6 to 7.5 ‰), intermediate (δ18O +8.5 to 9.0 ‰), and high δ18O (+11.0 to 12.0 ‰). The fourth is almost pure andradite with δ18O 10–12 ‰. Both the low and intermediate δ18O groups show significant variation in Fe content, whereas the two high δ18O groups are compositionally homogeneous. We interpret these features to indicate that the low and intermediate δ18O group garnets grew in separate fractionating magmas that were brought together through magma mixing, whereas the high δ18O 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 ~13 Ma to form hybridized Miocene granitoid magmas, leaving a refractory residue. The ~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. This is an excellent example of in situ crust–mantle hybridization in the deep Tibetan crust.