Identification of the early cretaceous granitic pluton and tectonic implications in the middle gangdese belt, southern tibet

Identification of the early cretaceous granitic pluton and tectonic implications in the middle gangdese belt, southern tibet

The Gangdese magmatic belt of southern Tibet is an ideal place to study Neo-.Tethyan subduction, continental crustal growth and reworking. However, there.are still controversies with regard to the evolution of the Neo-Tethys Ocean, the.magma source and the detailed diagenetic processes of igneous rocks in the.Gangdese belt. The Early Cretaceous magmatic exposures are sporadic in the.Gangdese magmatic belt. Thus the finding of the new exposure is key to.understanding the scenarios of the Neo-Tethys Ocean and geological.background of southern Tibet during the Early Cretaceous. In this.contribution, we undertook systematic geochronology, whole-rock.geochemistry and zircon Lu-Hf isotopic studies on a newly identified.granitic pluton in the middle Gangdese belt (Quesang area), southern Tibet..The results show that zircon U-Pb dating of three representative samples.yielded a weighted age of 120 ± 1.4 Ma, 117.3 ± 2.5 Ma and 114.0 ± 1.3 Ma,.respectively, which indicate the emplacement and crystallization age belonging.to the Aptian stage of the Early Cretaceous in response to the northward.subduction of the Neo-Tethyan oceanic lithosphere beneath the Lhasa terrane..In situ zircon Lu-Hf isotopic analyses indicate that ƐHf(t) values of the studied.granitic pluton are predominantly positive ranging from 7.2 to 11.4, and one.zircon shows negative ƐHf(t) value (−6.26), implying that a small amount of.ancient continental crustal materials might have participated in the magma.evolution. Geochemically, the granite samples are enriched in large ion.lithophile elements (LILE) and light rare earth elements (LREE), but depleted.in heavy rare earth elements (HREE), indicating arc-type geochemical.characteristics or subduction-related tectonic setting. In addition, combined.with mineral assemblages, zircon Lu-Hf isotopic features and low molar Al2O3/.(CaO+Na2O+K2O) (A/CNK) ratios of 0.91–1.10, the studied samples show a.close affinity with I-type granites. Moreover, zircon oxygen fugacity results.show that Ce4+/Ce3+ values range from 185 to 12, with a mean value of 78,.indicating a low oxygen fugacity setting similar to the Chile ore-barren granitic.plutons. In combination with published data, we argue that the Gangdese.magmatic belt may have developed continuous magmatism (145–105 Ma),and the notion of magmatic lull might deserve more consideration during the.Early Cretaceous. In this study, the Early Cretaceous granitic pluton might be the.result of the northward subduction of Neo-Tethys oceanic lithosphere beneath.the Lhasa terrane at a normal angle.

藏南冈底斯岩浆带是研究新特提斯洋俯冲、大陆地壳生长和再造的理想场所。然而，关于新特提斯洋的演化、岩浆源以及冈底斯带火成岩详细的成岩过程仍存在争议。早白垩世岩浆出露在冈底斯岩浆带中较为零星。因此，新出露岩体的发现对于理解早白垩世新特提斯洋的情况以及藏南的地质背景至关重要。在本文中，我们对藏南冈底斯带中部（确桑地区）一个新识别的花岗岩体进行了系统的年代学、全岩地球化学和锆石Lu - Hf同位素研究。结果表明，三个代表性样品的锆石U - Pb定年分别获得了120±1.4 Ma、117.3±2.5 Ma和114.0±1.3 Ma的加权年龄，这表明其侵位和结晶年龄属于早白垩世阿普特期，是对新特提斯洋岩石圈在拉萨地块之下向北俯冲的响应。原位锆石Lu - Hf同位素分析表明，所研究的花岗岩体的ƐHf(t)值主要为正值，范围从7.2到11.4，有一颗锆石显示出负的ƐHf(t)值（−6.26），这意味着少量古老大陆地壳物质可能参与了岩浆演化。在地球化学方面，花岗岩样品富集大离子亲石元素（LILE）和轻稀土元素（LREE），但亏损重稀土元素（HREE），显示出弧型地球化学特征或与俯冲相关的构造环境。此外，结合矿物组合、锆石Lu - Hf同位素特征以及低的摩尔Al₂O₃/(CaO + Na₂O + K₂O)（A/CNK）比值（0.91 - 1.10），所研究的样品与I型花岗岩具有密切的亲缘关系。而且，锆石氧逸度结果显示Ce⁴⁺/Ce³⁺值范围从185到12，平均值为78，表明其处于类似于智利无矿花岗岩体的低氧逸度环境。结合已发表的数据，我们认为冈底斯岩浆带可能发育了连续的岩浆作用（145 - 105 Ma），在早白垩世岩浆活动平静的观点可能需要更多的考虑。在本研究中，早白垩世花岗岩体可能是新特提斯洋岩石圈在拉萨地块之下以正常角度向北俯冲的结果。