A hybrid origin for two Cretaceous monzonitic plutons in eastern Zhejiang Province, Southeast China: Geochronological, geochemical, and Sr–Nd–Hf isotopic evidence

Abstract

Monzonites can provide important information about the nature of the mantle sources and the mechanism of crust–mantle interactions. However, details on the origin of Late Mesozoic monzonites in the Southeastern China remain poorly constrained. This paper presents whole-rock geochemical, Sr–Nd isotopic and zircon U–Pb and Hf isotopic data for two monzonitic plutons (Huangtanyang and Kanggu) in eastern Zhejiang Province, with the aim of elucidating their petrogenesis, and providing important insights into the process of crust–mantle interaction. LA-ICP-MS zircon U–Pb dating results imply that the Huangtanyang and Kanggu quartz monzonites were emplaced in Cretaceous (104–109Ma). All quartz monzonites are intermediate to acidic, metaluminous to weakly peraluminous, subalkaline, and K-rich in composition. They are enriched in large ion lithophile (e.g., Rb, Ba and Pb) and light rare earth elements, depleted in high-field strength elements (e.g., Nb, Ta, and Ti), and show weakly negative or no Eu anomalies (δEu=0.78–1.02). All quartz monzonites have homogeneous initial ISr values (0.7084–0.7090) and εNd(t) values (−7.50 to −6.84). They are characterised by highly variable zircon Hf isotopic compositions, with εHf(t) values ranging from −13.3 to −5.7. The combined geochemical evidences (such as high Mg# values, low Nb/U and Ta/U ratios, and variable zircon Hf isotopic compositions) suggests that both depleted asthenospheric and metasomatically enriched mantle components were involved in the formation of the monzonites. The existence of some zircons with unusually low εHf(t) values (low to −13.3) and Palaeoproterozoic two-stage Hf model ages from the Huangtanyang and Kanggu quartz monzonites also argues strongly for Palaeoproterozoic crustal involvement. Magma mixing played a dominated role in the genesis of these monzonites, as indicated by their wide range in zircon Hf isotopic compositions and the occurrence of mafic microgranular enclaves (MMEs). The MMEs show spheroidal to ellipsoidal–ovoidal shapes, and have igneous mineral assemblages and disequilibrium textures, such as acicular apatite and plagioclase with complex oscillatory zoning and repeated resorption surfaces, which indicate mingling/mixing of two different magmas. MMEs from Kanggu pluton have obviously similar trace element and Sr–Nd isotopic compositions to those of the host monzonites, with ISr values of 0.7089 and εNd(t) values of −7.16, however, they are distinct in having relatively enriched Sr and P, more depleted Zr and Hf, and having relatively depleted zircon Hf isotopic compositions with εHf(t) values varying from −8.4 to −3.8. We interpret that the Huangtanyang and Kanggu plutons have a mixed origin, i.e. partial melting of crustal materials triggered by upwelling of mantle-derived magma (including depleted asthenosphere and subduction-enriched mantle, represented by MMEs) under an extensional regime, and subsequent mixing of the mantle- and crust-derived melts resulted in the monzonites.