TY - JOUR

T1 - The origin of calcite in calc-silicate rocks from the Kokchetav ultrahigh-pressure metamorphic complex

AU - Mikhno, Anastasia O.

AU - Shatskiy, Anton F.

AU - Korsakov, Andrey V.

AU - Vinogradova, Yulia G.

AU - Berndt, Jasper

AU - Klemme, Stephan

AU - Rashchenko, Sergey V.

N1 - The research was supported by a Russian Science Foundation grant №22‐27‐00667, https://rscf.ru/project/22-27-00667/.

PY - 2024/2

Y1 - 2024/2

N2 - Understanding 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.

AB - Understanding 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.

KW - UHP metamorphism

KW - aragonite

KW - calcite

KW - hydrous carbonatitic melt

KW - inclusions

KW - metacarbonates

KW - trace elements

UR - https://www.scopus.com/record/display.uri?eid=2-s2.0-85175549745&origin=inward&txGid=263f4584e82674f45a52d9918fa3d8d2

UR - https://www.mendeley.com/catalogue/da82bd14-e7b6-3b82-b4fc-252befdad0d5/

U2 - 10.1111/jmg.12749

DO - 10.1111/jmg.12749

M3 - Article

VL - 42

SP - 143

EP - 170

JO - Journal of Metamorphic Geology

JF - Journal of Metamorphic Geology

SN - 0263-4929

IS - 2

ER -