Dolomitic marbles from the ultrahigh-pressure metamorphic Kimi complex in Rhodope, N.E. Greece

Abstract

Summary Dolomitic marbles from the Organi and Pandrosos areas of the ultrahigh-pressure (UHP) metamorphic Kimi complex in East Rhodope, N.E. Greece have the mineral as- semblage: Cal þ Dol þ Ol þ PhlDiHblSplTi-Chu þ retrograde Srp and Chl. Several generations of calcite and dolomite with variable composition and texture represent different stages of the P-T evolution: The first stage is represented by matrix dolomite (XMgCO3 ¼ 0.48) and relic domains of homogenous composition in matrix calcite (XMgCO3 ¼ 0.11-0.13); the second stage is evident from precipitation of lath- shaped and vermicular dolomite in matrix calcite. The third stage is represented by veinlets of almost pure CaCO3 and domainal replacement of prior calcite by nearly pure CaCO3 þ Ca-rich dolomite (XMgCO3 ¼ 0.34-0.43). Matrix dolomite adjacent to CaCO3 veinlets also becomes Ca-rich (XMgCO3 ¼ 0.42). In fact, Ca-rich dolomites with XMgCO3 in the range of 0.40-0.34 are reported for the first time from metamorphic marbles. Coexisting Ca-rich dolomite and Mg-poor calcite cannot be explained by the calcite- dolomite miscibility gap. This assemblage rather suggests that Mg-poor calcite was aragonite originally, which formed together with Ca-rich dolomite according to the reaction Mg-Cal ! Arg þ Dol (1) at ultrahigh pressures and temperatures above at least 850 � C, when dolomite becomes disordered and incorporates more Ca than coex- isting aragonite does in terms of Mg. The simplest explanation of these observations probably is to suggest two metamorphic events: The first one represented by relic matrix carbonates at relatively low to mod- erate pressures and temperatures of ca. 750 � C, and the second one limited by the mini- mum temperatures for dolomite disorder (ca. 850 � C) and in the aragonite þ dolomite