The origin of calcite in calc-silicate rocks from the Kokchetav ultrahigh pressure metamorphic сomplex

Mikhno, AO; Shatskiy, AF; Korsakov, AV; Vinogradova, YG; Berndt, J; Klemme, S; Rashchenko, SV

Abstract

Understanding calcite genesis in ultra-high 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 formation of calcite found in the ultrahigh pressure calc-silicate rocks from the Kokchetav massif. In the studied sample aragonite mineral inclusions coexist with Type A calcite inclusions (with the morphological features of 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 inclusion coexisting with aragonite inclusion in one garnet growth zone shows x(Ca)=0.935 implying their crystallization during a retrograde metamorphic stage at P ~ 2.3 GPa and T ~ 870 °C on the Kokchetav massif 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 pressure of inclusion capture exceeded 3.5 GPa, which contradicts the PT-conditions estimated by x(Ca) 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 =900-1000°C) and then re-equilibrated at lower conditions (P ≤ 2.3 GPa and T ≤ 870 °C). 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 show high LREE (6.8-64.9 ppm) concentrations along with the 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.