ABSTRACT
Intrusive calcareous bodies, marbles and calc-silicate rocks, are a distinctive feature of the high-grade metamorphic suites of the ~1 Ga northern Oaxacan Complex. They typically form dike-like intrusions up to 4 m thick which cut across the surrounding high-grade granulite- and upper-amphibolite facies metamorphic rocks. Various protoliths are possible for these carbonate bodies: (1) sediments including evaporites; (2) metasomatic skarns; and (3) carbonatites. An evaporitic protolith is supported by the predominance of scapolite, low abundances of incompatible trace elements (including Nb and rare-earth elements) relative to carbonatites, and the presence of a sharp contact with host rocks without a significant contact metamorphic aureole or fenitization. it is inferred that limestones and related rocks were remobilized under granulite-facies conditions and intruded into the host rocks. The widespread distribution of such evaporites in the Oaxacan Complex is consistent with deposition after the worldwide ~1.3 Ga oxygenation event that increased the marine sulfate reservoir. Intrusion of rift-related plutons into the sediments at ~1157–1130 Ma provides a younger limit on the age of protholiths of the metamorphic suites. Modern analogues for such evaporites are rifts associated with passive margins (e.g., Red Sea) and active margins (e.g., Gulf of California). The presence of evaporites implies a paleolatitude of 10–35°, a conclusion consistent with a paleo-geographic provenence for the Oaxacan Complex adjacent to either Amazonia or eastern Laurentia in Rodinia reconstructions.
