The origin of calcite in calc‐silicate rocks from the Kokchetav ultrahigh‐pressure metamorphic complex

Abstract

AbstractUnderstanding calcite genesis in ultrahigh‐pressure crustal rocks is a key to the reconstruction of the evolution of ultrahigh‐pressure metacarbonate rocks. Here, we present new data and a new model on the genesis and the P–T conditions of the formation of calcite found in the ultrahigh‐pressure calc‐silicate rocks from the Kokchetav massif. In the studied sample aragonite inclusions coexist with Type A calcite inclusions (previously interpreted as mineral inclusions) and the inclusions of Type B calcite (previously interpreted as derived from the crystallization of carbonatitic melt) in cores of garnet porphyroblasts. The most Mg‐rich calcite from Type A inclusions coexisting with aragonite inclusions in one garnet growth zone shows XCa = 0.935 implying their crystallization during a retrograde metamorphic stage at P ~ 2.3 GPa and T ~ 870°C along the P–T path. Type A calcite and aragonite inclusions were also found coexisting in one growth zone with K‐bearing clinopyroxene inclusion (ω[K2O] = 0.5 wt.%). Such a high K2O‐content in clinopyroxene testify that the pressure of inclusion capture exceeded 3.5 GPa, which contradicts the P–T conditions estimated by XCa in magnesian calcite. Thus, Type A calcite inclusions were initially captured as an aggregate of aragonite+ magnesian calcite at ultrahigh pressure metamorphic stage (P ≥ 3.5 GPa, T = 900–1,000°C) and then re‐equilibrated at lower conditions (P ≤ 2.3 GPa and T ≤ 870°C). The trace element composition of aragonite and Type A and Type B calcite from inclusions was also studied to clarify calcite genesis in these inclusions. Aragonite shows high LREE (5–57 ppm) and Sr‐content (600–800 ppm). Calcite from Type A inclusions shows low LREE (2.9–19.8 ppm) and Sr‐content (490–670 ppm). Calcite from Type B inclusions forms two groups according to the LREE and Sr content distribution (Type B1 and Type B2). Trace element distribution in Type B1 calcite is identical to that of Type A calcite, while Type B2 calcite shows high LREE (6.8–64.9 ppm) concentrations along with low Sr‐content (180–340 ppm). Type A and Type B1 calcite is interpreted to have been re‐equilibrated. Type B2 calcite inclusions crystallized from the hydrous carbonatitic melt.