西北大学朱韧之课题组取得一项新突破。他们的最新研究揭示了中国拉萨地块东南部晚白垩世高Ba-Sr花岗闪长岩的岩石成因，并提出了幼年地壳改造和大陆生长的意义。这一研究成果发表在2024年6月23日出版的国际学术期刊《地球化学学报》上。

研究显示，拉萨地块东南部晚白垩世高Ba-Sr花岗闪长岩的识别，可能为研究青藏高原大陆演化提供有价值的信息。锆石U-Pb年龄数据显示，花岗闪长岩的侵位年龄在87.32±0.43Ma。高Ba-Sr花岗闪长岩的地球化学特征为K₂O+Na₂O含量(8.18-8.73wt%)和K₂O/Na₂O比值(0.99-1.25，大多数＞1)，属于高钾钙碱性至钾玄岩系列。Yonglaga花岗闪长岩的Sr (653-783 ppm)和Ba (1346-1531 ppm)含量高，Sr/Y(30.92-38.18)和(La/Yb)_N(27.7-34.7)比值高，但Y (20.0-22.8 ppm)和Yb (1.92-2.19 ppm)含量较低，且无Eu负异常(δEu=0.83-0.88)，具有典型的高Ba-Sr花岗类特征。

可变锆石εHf(t)值为-4.58至+12.97，初始⁸⁷Sr/⁸⁶Sr同位素比值升高至0.707254-0.707322，εNd(t)值降低至-2.8至-3.6与Hf体系解耦表明，交代地幔源中含有大量的再循环古代物质。这些高Ba-Sr元素的出现表明了交代幔源幼年物质对大陆的前期贡献，暗示了拉萨东南部晚白垩世大陆地壳的生长。结合区域数据，研究人员推断，底侵基性岩浆提供的大量的热量是导致幼年的部分熔融的主要原因。交代地幔的熔融可以形成一个年轻的基性下地壳，其中高Ba-Sr花岗岩类是对之前的拉萨地块东南部晚白垩世(100–80Ma)的基性地壳改造而来。

附：英文原文

Title: Petrogenesis of late Cretaceous high Ba–Sr granodiorites, SE Lhasa block, China: implications for the reworking of juvenile crust and continental growth

Author: Lin, Li-Hang, Zhu, Ren-Zhi, Lai, Shao-Cong, Qin, Jiang-Feng, Zhu, Yu, Zhao, Shao-Wei, Liu, Min

Issue&Volume: 2024-06-23

Abstract: The high Ba–Sr rocks can provide significant clues about the evolution of the continent lithosphere, but their petrogenesis remains controversial. Identifying the Late Cretaceous high Ba–Sr granodiorites in the SE Lhasa Block could potentially provide valuable insights into the continent evolution of the Qinghai-Tibet Plateau. Zircon U–Pb ages suggest that the granodiorites were emplaced at 87.32±0.43 Ma. Geochemically, the high Ba–Sr granodiorites are characterized by elevated K₂O+Na₂O contents (8.18–8.73 wt%) and K₂O/Na₂O ratios (0.99–1.25, mostly>1), and belong to high-K calc-alkaline to shoshonitic series. The Yonglaga granodiorites show notably high Sr (653–783 ppm) and Ba (1346–1531 ppm) contents, plus high Sr/Y (30.92–38.18) and (La/Yb)N (27.7–34.7) ratios, but low Y (20.0–22.8 ppm) and Yb (1.92–2.19 ppm) contents with absence of negative Eu anomalies (δEu=0.83–0.88), all similar to typical high Ba–Sr granitoids. The variable zircon εHf(t) values of-4.58 to+12.97, elevated initial ⁸⁷Sr/⁸⁶Sr isotopic ratios of 0.707254 to 0.707322 and low εNd(t) values of-2.8 to-3.6 with decoupling from the Hf system suggest that a metasomatized mantle source included significant recycled ancient materials. The occurrence of such high Ba–Sr intrusions indicates previous contributions of metasomatized mantle-derived juvenile material to the continents, which imply the growth of continental crust during the Late Cretaceous in the SE Lhasa. Together with regional data, we infer that the underplated mafic magma provides a significant amount of heat, which leads to partial melting of the juvenile crust. The melting of the metasomatized mantle could produce a juvenile mafic lower crust, from which the high Ba–Sr granitoids were derived from reworking of previous mafic crust during the Late Cretaceous (ca. 100–80 Ma) in the SE Lhasa.

DOI: 10.1007/s11631-024-00708-z

Source: https://link.springer.com/article/10.1007/s11631-024-00708-z