Petrology, geochemistry, and crystal size distribution of the basaltic andesite–dacite association at Mt. Sumbing, Central Java, Indonesia: Insights to magma reservoir dynamics and petrogenesis

Abstract

Ten rock samples consisting of one pyroclastic density current (PDC1) deposit, seven lava flows (LF1–7), and two summit lava domes (LD1, 2) were studied to understand the petrogenesis and magma dynamics at Mt. Sumbing. The stratigraphy is arranged as LF1, PDC1, LF2, LF3, LF4, LF5, LF6, LF7, LD1, and LD2; furthermore, these rocks were divided into two types. Type I, observed in the oldest (LF1) sample, has poor MgO and high Ba/Nb, Th/Yb and Sr. The remaining samples (PDC1–LD2) represent type II, characterized by high MgO and low Ba/Nb, Th/Yb and Sr values. We suggest that type I is derived from AOC (altered oceanic crust)-rich melts that underwent significant crustal assimilation, while type II originates from mantle-rich melts with less significant crustal assimilation. The early stage of type II magma (PDC1–LF3) was considered a closed system, evolving basaltic andesite into andesite (55.0–60.2 wt% SiO₂) with a progressively increasing phenocryst (0.30–0.48 \(\upphi_{PC}\)) and decreasing crystal size distribution (CSD) slope (from − 3.9 to − 2.9). The evidence of fluctuating silica and phenocryst contents (between 55.9–59.7 wt% and 0.25–0.41 \(\upphi_{PC}\), respectively), coupled with the kinked and steep (from − 5.0 to − 3.3) CSD curves imply the interchanging condition between open (i.e., magma mixing) and closed magmatic systems during the middle stage (LF4–LF6). Finally, it underwent to closed system again during the final stage (LF7–LD2) because the magma reached dacitic composition (at most 68.9 wt% SiO₂) with abundant phenocryst (0.38–0.45 \(\upphi_{PC}\)) and gentle CSD slope (from − 4.1 to − 1.2).