Abstract
The precise timing, petrogenesis, and geodynamic significance of three granitoid bodies (Beidao granite, Caochuanpu granite, Yuanlongzhen granite, and the Roche type rock) of the Tianshui area in the Western Qinling Orogen, central China, are poorly constrained. We performed an integrated study of petrology, geochemistry, and zircon U-Pb dating to constrain their genesis and tectonic implication. Petrographic investigation of the granites shows that the rocks are mainly monzogranites. The Al saturation index (A/CNK versus SiO2) of the granitoid samples indicates meta-aluminous to peraluminous I-type granites. Their magmas were likely generated by the partial melting of igneous protoliths during the syn-collisional tectonic regime. Rare-earth-elements data further support their origin from a magma that was formed by the partial melting of lower continental crust. The Beidao, Caochuanpu, and Yuanlongzhen granites yielded U-Pb zircon weighted mean ages of 417 ± 5 Ma, 216 ± 3 Ma, and 219 ± 3 Ma, respectively. This study shows that the Beidao granite possibly formed in syn- to post-collision tectonic settings due to the subduction of the Proto-Tethys under the North China Block, and can be linked to the generally reported Caledonian orogeny (440–400 Ma) in the western segment of the North Qinling belt, whereas Yuanlongzhen and Caochuanpu granites can be linked to the widely known Indosinian orogeny (255–210 Ma). These granitoids formed due to the subduction of the oceanic lithospheres of the Proto-Tethyan Qinling and Paleo-Tethyan Qinling. The Roche type rock, tourmaline-rich, was possibly formed from the hydrothermal fluids as indicated by the higher concentrations of boron leftover during the late-stages of magmatic crystallization of the granites.
Highlights
The Qinling Orogenic Belt (QOB) separates the North China Block (NCB) from the SouthChina Block (SCB) [1,2,3,4,5,6] and is considered to have formed by the long-term convergence between the NCB and SouthChina Block (SCB) (Figure 1a) [2,4,7,8,9]
Three main types of granitoids (i.e., Beidao granite, Yuanlongzhen granite, Caochuanpu granite) and the Roche type rocks were investigated in this study
(3) Geochemical data further classify them as magnesian (Beidao granite), ferroan to magnesian types (Caochuanpu and Yuanlongzhen)
Summary
The Qinling Orogenic Belt (QOB) separates the North China Block (NCB) from the SouthChina Block (SCB) [1,2,3,4,5,6] and is considered to have formed by the long-term convergence between the NCB and SCB (Figure 1a) [2,4,7,8,9]. The Qinling Orogenic Belt (QOB) separates the North China Block (NCB) from the South. The QOB is a composite structure that has undergone a complex history of rifting, oceanic subduction, continent-continent collision, and intracontinental orogenic deformation [10]. It has experienced four tectono-thermal events during the Neoproterozoic, Paleozoic, Early, and Late Mesozoic, respectively, equivalent to four periods of granitoid intrusions [11]. Two suture zones marked by ophiolite complexes (the Shangdan Suture Zone and the Mianlüe Suture Zone, Figure 1a) are related to orogenic events that occurred during the Devonian and Late Triassic, respectively [2,10,12,13,14]
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