矿床地球化学国家重点实验室知识库

A study using whole-rock major-trace elements and Sr–Nd isotopes as well as zircon U–Pb dating has been carried out on Early Cretaceous mafic–ultramafic intrusions from the Gan–Hang tectonic belt (GHTB), South China, to understand the origin of mantle sources and the sequential evolution of the underlying Late Mesozoic lithospheric mantle of this area. The study focused on two intrusions, one at Quzhou and the other at Longyou (see Fig. 1). They are primarily composed of mafic–ultramafic rocks with wide range of chemical compositions. The Quzhou mafic rocks have relatively narrow ranges of SiO₂ (48.94–51.79 wt%), MgO (6.07–7.21 wt%), Fe₂O₃ (10.48–11.56 wt%), CaO (8.20–8.81 wt%), and Mg^# (51.7–56.5) with relatively low K₂O (0.56–0.67 wt%) and Na₂O (3.09–3.42 wt%). By contrast, the ultramafic rocks from Longyou have distinct lower SiO₂ (41.50–45.11 wt%) and higher MgO (9.05–9.90 wt%), Fe₂O₃ (12.14–12.62 wt%), CaO (8.64–10.67 wt%), and Mg^# (59.5–61.1) with relatively higher K₂O (1.32–1.75 wt%) and Na₂O (4.53–5.08 wt%). They are characterized by Ocean Island Basalts (OIB)-type trace element distribution patterns, with a significant enrichment of light rare earth elements (LREE), large ion lithophile elements (LILE, i.e., Rb, Ba, K, and Sr) and high field strength elements (HFSE, i.e., Nb, Ta), and slight depletion of Th, U, Ti, and Y. The intrusions exhibit relatively depleted Sr–Nd isotope compositions, with (⁸⁷Sr/⁸⁶Sr)_irange of 0.7035 to 0.7055 (¹⁴³Nd/¹⁴⁴Nd)_i of 0.51264 to 0.51281 and ε_Nd(t) values of + 3.0 to + 6.6. Zircon U–Pb dating of Longyou and Quzhou intrusions yields consistent magma emplacement ages of 129.0 ± 3.9 to 126.2 ± 2.4 Ma, respectively. The dating results are consistent with the peak of extension in Early Cretacerous throughout the Gan–Hang tectonic belt. Their magmas were principally derived from near-solidus partial melting of pyroxenites with different content of silica, and the pyroxenites were resulted from a juvenile SCLM peridotite metasomatized by adakitic and felsic melts under a subducted oceanic crust-lithospheric mantle in Neoproterozoic. During the subduction of the oceanic crust, the juvenile SCLM can be sandwiched by the ancient SCLM above and the pyroxenites below and stored for hundreds of millions of years until Late Mesozoic. Then partial melting of the pyroxenites was triggered during the peak of extension in response to the tectonic reactivation of the GHTB in Early Cretaceous. It is clear that fault reactivation and structural constraints within the GHTB had played an important role in its magmatic evolution.