Early Silurian granitic rocks and associated enclaves as evidence of rapid cooling in a cognate magma system: the case of the Xuehuading–Panshanchong pluton, South China Block

Abstract

The study of enclaves in granitic plutons provides fundamental information on the petrogenesis of their host rocks. Here we combine U–Pb zircon ages, petrography, geochemistry and Nd–Hf isotope composition to investigate the origin of dioritic–granodioritic enclaves and their host granodiorites and biotite granites in the Xuehuading–Panshanchong area, which is a pivotal site to study the Palaeozoic intracontinental orogenic processes of the South China Block. Obtained ages indicate that the host rocks were formed in early Silurian time (c. 432 Ma). The enclaves are fine grained, but with mineral assemblages similar to their hosts and contain amphibole, biotite and plagioclase. All rocks have fractionated rare earth element patterns ((La/Yb)N = 2.86–8.16), except for one biotite granite that has a concave rare earth element pattern ((La/Yb)N = 1.50). Most rocks are depleted in Ta–Nb–Ti, and have negative Eu anomalies and ϵNd(t) (–8.86 to –5.75) and zircon ϵHf(t) (–13.30 to –4.11, except for one, –39.08). We interpret that the enclaves were formed at the borders of magma-ascending conduits, where the mafic mineral crystallization was enhanced by rapid cooling. Conversely, the biotite granites were produced by fractional crystallization from a related granodiorite magma. The sample with a concave rare earth element pattern may have been influenced by hydrothermal fluid–melt interaction. Geochemical modelling suggests that the granodiorites were likely generated by disequilibrium melting of heterogeneous amphibolites in the middle–lower crust. Considering the geological data for the Palaeozoic magmatic rocks in the South China Block, we propose that the Xuehuading–Panshanchong magmatism was likely triggered by piecemeal removal of the thickened lithospheric root and subsequent thermal upwelling of mantle, without a mantle-derived magma contribution to the granites.

Two Distinct Fractional Crystallization Mechanisms of A-Type Granites in the Nanling Range, South China: A Case Study of the Jiuyishan Complex Massif and Xianghualing Intrusive Stocks

“…Granitic rocks, diorite and mafic rocks are widely distributed in this region (Figure 1b). Early Paleozoic granites are commonly S-and I-type granites; they are less abundant and formed large intrusions, corresponding to the Caledonian intracontinental orogenic event in South China [31]. Early Mesozoic granites are also S-and I-type granites; they are the least common and formed small intrusions, triggered by the amalgamation of the North China Craton and the South China Block in the Triassic period [21].…”

“…Early Mesozoic granites are also S-and I-type granites; they are the least common and formed small intrusions, triggered by the amalgamation of the North China Craton and the South China Block in the Triassic period [21]. Late-Mesozoic granites are most commonly developed in this area, which are usually A-type granites and accompanied with cotemporaneous basalts and diorites, corresponding to the intraplate extension triggered by the roll-back of subducted paleo-pacific plates in the Jurassic period [31].…”

“…Each unit is distributed from west to east with different shapes and lithologies. The westernmost unit, called Xuehuading (Figure 2), is composed of granodiorite, monzonitic granite, and biotite granite, with zircon U-Pb ages of between 432-412 Ma [31][32][33], and was formed much earlier than the other four units. Abundant, dark enclaves were discovered in the Xuehuading unit.…”

“…These MMEs are dark gray, with sizes ranging from 5-10 cm. The MMEs commonly occur as lobate shapes and do not show evidence of any reaction with the host magma, and were likely formed by mafic mineral aggregation during magma ascent rather than mantle magma injection [31]. The Xuehuading granite has protoliths of Paleoproterozoic metaigneous and metapelitic rocks, with an I-type affinity [33].…”

Cambrian and earliest Ordovician fauna and geology of the Sông Đà and adjacent terranes in Việt Nam (Vietnam)

“…This scenario may reconcile palaeontological evidence with structural data from the Sông Mã suture. Evidence of early Palaeozoic magmatism and thermal events across various parts of what was to become Indochina (Roger et al 2000(Roger et al , 2007Carter & Clift, 2008;Zelázniewicz et al 2013;Shi et al 2015;Metcalfe, 2017) might relate in some way to the 'Guangxi' or 'Kwangsian' event (Carter et al 2001;Ou et al 2021) that is also well represented in the eastern part of South China (Xu et al 2016). Zhan et al (2010) noted that the Cambrian strata of the Guole area of Guangxi Province, which borders northernmost Việt Nam, comprise shallow-water carbonate facies, although with respect to the general palaeogeography of South China, the area is surrounded by slope facies.…”