Ultrahigh-Pressure Orange Garnets from the Sedimentary Deposits of the Upper Bug Area (Ukrainian Shield)

Abstract

The article is dedicated to the investigation of orange garnets from Miocene-Pliocene deposits of the Balta suite and the Sarmatian stage in the Vinnytsia area. The Vinnytsia area is located on the southwestern slope of the Ukrainian Shield. The crystalline basement on most of the area is represented by granitoids of the Berdychiv complex of Paleoproterozoic age, and to a lesser extent by enderbites and charnockites of the Lityn complex of Neoarchean age. Among these, there are mafic and ultramafic bodies ranging from 0.5 to 2 meters in thickness. The sedimentary cover with a total thickness of up to 20 m is represented by sands and clays of the middle Sarmatian, Balta and Quaternary sediments. These deposits are enriched with mantle minerals, predominantly pyrope, and four diamond crystals have also been discovered. We studied the chemical composition (253 crystals) and morphology (278 crystals) of the orange garnets. The studied samples are mostly represented by garnets of pyrope and pyrope-almandine composition, a small part is represented by grossulars. Most of the garnets are unrounded crystal fragments ranging in size from 0.2 to 0.6 mm, exhibiting signs of intense hypergene changes. These are evident in the development of positive (block) and negative relief features, appearing as drop-shaped, cone-shaped, small hilly microrelief and various forms of etching channels. The morphological features suggest that these garnets were probably relocated a short distance from their primary sources and had been present in the weathering crust for an extended period. The original sources of garnets are most likely Precambrian and located on active tectonic zones, which led to their destruction. According to chemical composition data, among orange garnets low-chromium websterite and low-calcium eclogite varieties, as well as garnets of high-iron mantle pyroxenites are dominated. The presence of titanium impurities in many garnets indicates the possible diamond-bearing nature of some primary sources.