High-Mg andesitic rocks formed through crustal magmatic differentiation

High-Mg andesitic rocks are enriched in both compatible (Mg, Cr and Ni) and incompatible elements (Si, Ba and Sr), which indicates the complexity of their petrogenesis. Models involving hybridization of the mantle source and input from adakitic melts have been invoked, but struggles to explain the low Mg# values (<0.65) of some low Sr/Y (<40) high-Mg andesitic rocks , because these rocks cannot represent primary melts (Mg# ≥ 0.72) produced by partial melting of mantle peridotite. A petrological and geochemical analysis of the Xinsi diorite (443±3 Ma) and the Tiechang gabbro (440±3 Ma) from the Early Paleozoic Wuyi-Yunkai Orogen in South China is presented to show the role of magmatic differentiation in petrogenesis of high-Mg andesite. These rocks have high SiO₂, MgO, Ba and Sr concentrations with low Mg# values (≤ 0.69) and Sr/Y ratios (≤ 16.7), resembling typical low Sr/Y high-Mg andesites. Amphiboles and clinopyroxenes in the Xinsi diorite contain variable contents of Ba and Sr, which are negatively correlated with Mg#. This indicates that their parental melts had low Ba and Sr and underwent significant enrichment of Ba and Sr through the fractionation of Mg-rich minerals at the upper crust (3.1–4.4 kbar). Clinopyroxenes, plagioclases and biotites in the Tiechang gabbro all have lower Ba and Sr than those in the Xinsi diorite, suggesting that their parental melts were also initially low in Sr and Ba and experienced enrichment of these elements during magma evolution at middle crustal levels (5.7–6.3 kbar). Low Ba-Sr and Sr/Y for parental melts of the Xinsi diorite and Tiechang gabbro suggest their derivation from the partial melting of mantle sources with input from slab melts at depths shallower than the garnet stability field. This study demonstrates the importance of fractional crystallization in the genesis of low Sr/Y high-Mg andesitic rocks and is conflict with past views that their geochemical features were related to the involvement of adakitic melts in their mantle sources. Therefore, this study presents an alternative model for the production of low Sr/Y and high Sr/Y high-Mg andesitic rocks, particularly in the Late Archean, which reflects intense melting of subducted slab at different depths in the hotter mantle regime at that time.