Search

[Display omitted] • The Nd and Hf isotopes are decoupled in the Early Cretaceous rocks of SQB. • Partial melting of subduction mélange induced the Nd-Hf isotopic decoupling. • Mélange is an important medium for chemical exchange between crust and mantle. Subduction zones are the main sites of material exchange between the crust and mantle. Sediments subducted at these zones are widely considered to play a key role in generating arc magmas. Here we report whole-rock Nd–Hf isotopic compositions of late Mesozoic intermediate–felsic igneous rocks from the southern Qiangtang Block, central Tibet. The late Jurassic Rena Co (ca. 150 Ma) and Jiacuo (ca. 152 Ma) granodiorites , which were derived from ancient lower crust, show Nd–Hf isotopic coupling (Δε Hf (t) = 0.7–2.4). Conversely, the early Cretaceous Rena Co granodiorite porphyries (ca. 112 Ma), which were derived from juvenile lower crust, display marked Nd–Hf isotopic decoupling (Δε Hf (t) = 7.5). We suggest that the juvenile crust was compositionally andesitic and similar to the Duolong andesitic rocks (118–106 Ma) of the southern Qiangtang Block, which were derived by partial melting of the mélange (a physical mixture of mid-ocean ridge basalts, subducted sediments, and mantle-wedge peridotites). A simple mixing modeling indicates that the source of the early Cretaceous Duolong andesitic rocks includes 1–2% bulk subducted sediments. The Duolong andesitic rocks also show Nd–Hf isotopic decoupling (Δε Hf (t) = 3.0–11.3). Furthermore, the early Cretaceous Duolong dioritic and granitic rocks, which were generated by mixing between andesitic melts and ancient-lower-crust-derived magmas, display Nd–Hf isotopic decoupling. Thus, we infer a significant contribution of mélange melting in the Nd–Hf isotopic decoupling of early Cretaceous igneous rocks of the southern Qiangtang Block. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Mikengshan-Qingxixiang-Yanbei igneous rocks were formed at 145–134 Ma. • The Yanbei Sn-bearing granite porphyries were derived from ancient crustal rocks. • Reduced redox state and ancient crustal sources benefit for Sn mineralization. A world-class Sn province in South China contains granite- and porphyry-related Sn deposits. In this paper, we investigate the source of Sn, based on a study of Early Cretaceous (145–134 Ma) ore-barren granites and granite porphyries, ore-barren rhyolites, and Sn-bearing granite porphyries in the Mikengshan (MKS)–Qingxixiang (QXX)–Yanbei (YB) area of South China. The ore-barren rhyolites have enriched whole-rock Nd and zircon Hf isotopic compositions [ε Nd (t) = − 9.1 to − 10.5; ε Hf (t) = − 5.9 to − 13.8]. The ore-barren granites and granite porphyries have A-type granite characteristics and less enriched Nd–Hf isotopic compositions [ε Nd (t) = − 4.9 to − 3.2; ε Hf (t) = − 6.0 to + 1.7] than the ore-barren rhyolites. The Sn-bearing granite porphyries have similar rare earth element and trace element patterns to the ore-barren rhyolites. In addition, they have similar Nd but less enriched Hf isotopic compositions [ε Nd (t) = − 8.9; ε Hf (t) = − 3.3 to + 0.4] compared with the ore-barren rhyolites. The average ΔFMQ values of the studied volcanic (–0.50) and intrusive (–0.21) rocks indicate reduced condition during magma evolution. We suggest that the ore-barren rhyolites were likely derived by partial melting of Paleoproterozoic to Mesoproterozoic crustal metasedimentary rocks and that the Sn-bearing granite porphyries were possibly formed by partial melting of Paleoproterozoic metasomatized mantle-derived crustal rocks. Whereas, the ore-barren granites and granite porphyries were likely generated by partial melting of a hybridized source consisting of ancient and late Mesozoic juvenile crustal rocks. We propose that the reduced redox state and ancient crustal sources played an important role for Sn mineralization in the MKS–QXX–YB area. [ABSTRACT FROM AUTHOR]

• Granodioritic pluton and dike were formed at 153–152Ma in southern Qiangtang block. • The Jurassic granodiorites were derived by partial melting of ancient mafic lower crust. • Jurassic magmatism in southern Qiangtang block was produced in an arc setting. A Late Jurassic–Late Cretaceous magmatic belt extends for >800 km along the southern margin of the southern Qiangtang Block (SQB) in central Tibet. However, the Mesozoic tectonic setting of these igneous rocks remains uncertain. Here, we report new in situ zircon U–Pb ages and Hf–O isotopes as well as whole-rock major- and trace-element compositions and Sr–Nd isotopes for the Jiacuo granodiorites and dikes in the SQB. Zircon laser ablation–inductively coupled plasma–mass spectrometry U–Pb dating indicates that these rocks were emplaced during the Late Jurassic (ca. 153–151 Ma). All of the rocks have high SiO 2 (64.7–68.4 wt%) and K 2 O (4.2–5.2 wt%) contents and are characterized by enrichment in light rare-earth elements ((La/Yb) N = 16–19) and large-ion lithophile elements, and depletion in high-field-strength elements. The granodiorites and dikes have variable zircon Hf–O isotope values (ε Hf (t) = −15.7 to −10.1, δ18O = +5.32‰ to +7.88‰), but uniform and high whole-rock initial 87Sr/86Sr isotopic (0.7079 to 0.7080) and low ε Nd (t) (−7.9 to −7.6) values. We suggest that these granodiorites were originated by partial melting of ancient mafic lower crust of the SQB. Combining our new data with the temporal and spatial distributions of Mesozoic magmatic rocks in the SQB, we propose that Late Jurassic magmatism in the SQB was resulted from northward subduction of the Bangong–Nujiang Tethyan oceanic slab during this period. [ABSTRACT FROM AUTHOR]