Heavymagnesium isotopes in the Gangdese Magmatic Belt: Implications for magmatism in the Mesozoic subduction system of southern Tibet

The Gangdese magmatic belt of southern Tibet preserves important records of the Mesozoic Neo-Tethyan orogeny..Here we investigate a newly identified Late Cretaceous hornblende gabbro pluton using zircon U–Pb geochronology,.whole-rock major and trace elements and Sr-Nd-Hf-O-Mg isotopes with a view to evaluate the.magma source characteristics and fluid evolution. The geochronological data and Sr-Nd-Hf isotopes suggest.that the Late Cretaceous gabbro crystallized in the Cenomanian stage of the Late Cretaceous (ca. 98 Ma), from.magma generated by partial melting of the depleted mantle. We report the first set of Mg isotopic data for the.Late Cretaceous and Late Triassic arc-type gabbroid rocks in the Gangdese magmatic belt, where both rocks.show homogeneous heavy Mg isotopic compositions with δ26Mg values ranging from 0.01 to 0.12‰ and −.0.09 to 0.09‰, respectively. Their δ26Mg values are higher than the average mantle values (−0.25 ± 0.07‰),.suggesting that 26Mg-rich fluids were involved in their mantle sources. Based on Sr-Nd-Hf-O isotopic data and.published information on Mg isotopic reservoir, we argue that talc-rich serpentinite fluids played an important.role in producing the heavy Mg isotopes in these arc-related intrusions. Integrating data from previous studies.and this study, we propose that the northward subduction of the Neo-Tethys oceanic lithosphere and formation.of arc-type magmas provide a viablemodel to account for the heavy Mgisotopic compositions through fluid-rock.interaction within the subduction channel. This further indicates that the northward subduction of the.Neo-Tethys oceanic lithosphere commenced in the Late Triassic or earlier rather than in the Early Cretaceous.(ca. 145 Ma).

藏南冈底斯岩浆带保存着中生代新特提斯造山运动的重要记录。在此，我们利用锆石U - Pb年代学、全岩主量和微量元素以及Sr - Nd - Hf - O - Mg同位素，对一个新确定的晚白垩世角闪辉长岩岩体进行研究，以评估岩浆源特征和流体演化。年代学数据和Sr - Nd - Hf同位素表明，晚白垩世辉长岩在晚白垩世塞诺曼期（约98 Ma）结晶，源自亏损地幔部分熔融产生的岩浆。我们报道了冈底斯岩浆带晚白垩世和晚三叠世弧型辉长岩类岩石的第一组Mg同位素数据，这两种岩石都显示出均一的重Mg同位素组成，其δ26Mg值分别在0.01‰到0.12‰以及 - 0.09‰到0.09‰之间。它们的δ26Mg值高于地幔平均值（ - 0.25±0.07‰），这表明其地幔源区涉及富26Mg的流体。基于Sr - Nd - Hf - O同位素数据以及已发表的Mg同位素储库信息，我们认为富滑石的蛇纹岩流体在这些与弧相关的侵入体产生重Mg同位素过程中起到了重要作用。综合先前研究和本研究的数据，我们提出新特提斯大洋岩石圈向北俯冲以及弧型岩浆的形成提供了一个合理的模型，通过俯冲通道内的流体 - 岩石相互作用来解释重Mg同位素组成。这进一步表明新特提斯大洋岩石圈向北俯冲始于晚三叠世或更早，而非早白垩世（约145 Ma）。