Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust–mantle mixing and metamorphism in the deep crust

Abstract

juvenile character (εHf 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 δ 18 O (+8.73-8.97 ‰) values, and extremely low εHf 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 Abstract 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.% TiO 2 and ~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 Communicated by Hans Keppler. Contrib Mineral Petrol (2016) 171:62 1 3 62 Page 2 of 19 solutions and have low δ 18 O (+6 to 7.5 ‰), intermediate (δ 18 O +8.5 to 9.0 ‰), and high δ 18 O (+11.0 to 12.0 ‰). The fourth is almost pure andradite with δ 18 O 10-12 ‰. Both the low and intermediate δ 18 O groups show significant variation in Fe content, whereas the two high δ 18 O groups are compositionally homogeneous. We interpret these features to indicate that the low and intermediate δ 18 O group garnets grew in separate fractionating magmas that were brought together through magma mixing, whereas the high δ 18 O 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 Electronic supplementary material The onli...

No mafic layer in 80 km thick Tibetan crust

“…4), calculated by the thermodynamic, phase equilibrium code, Perple_X 36,37 , for relevant temperature regimes 38 . The chosen 39,40 as well as pelite granulite 1 and a global average continental lower crust 41 .…”

“…Calculations were carried out with the Perple_X_6.8.5 phase equilibrium modeling program36 for the internally consistent thermodynamic dataset ds6237 . The composition of rocks is chosen according to xenoliths (felsic granulite and mafic-intermediate granulite) in southern Tibet39,40 as well as a pelite granulite 1 and a global average lower crustal model 41 . a Calculated geotherms for different assumed heat flow values.…”

Miocene multi‐source magma system and magma mixing in the Zhunuo porphyry Cu deposit, southern Tibet, China

“…The Zr, P 2 O 5 , and TiO 2 contents are inversely correlated with SiO 2 (Figure 5b,c), indicating zircon, apatite, and Ti‐bearing mineral had been fractionated from the magma or remained as residues in the source. On the whole, the quartz syenite‐normal granite porphyry magma is likely derived from plagioclase, ilmenite, zircon, apatite bearing source of ancient middle‐upper Lhasa crust, which the mineral assemblages are different from the acid granulite enclaves (Chan et al, 2009; Wang et al, 2016) of ancient Lhasa lower crust (Wang et al, 2016).…”

Emplacement of Young Island Arc Crust Over Older Mantle Along a Cratonic Foreland: Constraints From Multiple Isotopes and Elemental Geochemistry

“…Meanwhile, increasing studies have revealed that mantle‐derived mafic rocks are sometimes more isotopically enriched relative to the coexisting crustal felsic rocks, contrary to “standard” behavior (e.g., Jiang et al, 2018, and references therein; Shen et al, 2018). Furthermore, magmatic systems with such features commonly occur in regions associated with island arc accretion, such as the Central Asian Orogenic Belt (CAOB) (e.g., Huang et al, 2013; Shen et al, 2009, 2012) and Gangdese Belt (South Tibet) (e.g., Lu et al, 2019; Meng et al, 2019; R. Wang, Richards, Zhou, et al, 2015; Wang et al, 2016; Wang, Collins, et al, 2017; Wang, Weinberg, et al, 2018), but the relationship between the two processes (isotope inversion and island arc accretion) has not been clarified. Recently, Jiang et al (2018) have recognized an “reversed isotope” case in Gan‐Hang Belt (GHB) (South China), where the Neoproterozoic‐aged island arc rocks were locally exposed (Shu et al, 2019; Xia et al, 2018; Yao et al, 2019).…”