Does the South Apuseni Mts. ophiolitic nappe extend eastward up to the east Carpathians? New data on volcanics from the Transylvanian Depression and Transylvanian nappes (Romania)

The South Apuseni Mountains (SAM) are characterized by the widespread occurrence of Middle Jurassic ophiolitic sequences, generated in a mid-ocean ridge setting, and covered by Late Jurassic calc-alkaline volcanics originated in an intra-oceanic island arc. Since 1975, geophysical data have suggested that the SAM ophiolitic nappe extend eastward below the Neogene sedimentary cover of the Transylvanian Depression. In addition, mafic rocks were recovered in deep wells below the Transylvanian Depression; most of these are not studied thoroughly with the exception of some calc-alkaline volcanics. The Transylvanian Nappes are located in the inner part of East Carpathians and are commonly interpreted in literature as having their roots in the ophiolitic nappe below the Transylvanian Depression; in other words, these nappes are commonly interpreted as the easternmost end of the SAM ophiolites. The Transylvanian Nappes include the Olt (Persani), Haghimas, and Rarau Nappes, which consist of various units including a Barremian-lower Aptian wildflysch with blocks of Triassic and Jurassic limestones, arenites, and volcanic rocks of various nature: calc-alkaline, tholeiitic and alkaline. This wildflysh lies at the top of an Early Cretaceous flysch belonging to the sedimentary cover of a crystalline tectonic unit (Bucovinian Nappe), and is covered by Miocene and Quaternary sedimentary and volcanic formations. In this work volcanic rocks from a deep well in the Transylvanian Depression and from the Olt Nappe are studied in order to constrain their possible relationships with the SAM ophiolitic nappe. Samples of volcanic rocks from the Transylvanian Depression were collected from the Deleni-6042 deep well made by Romgaz in the Deleni area, between the depth of 4742 and 5017m. The Olt Nappe represent the southernmost outcrop of the Transylvanian Nappes, and the geochemistry and nature of its volcanics is still poorly known. These volcanics basically crop out in three distinct areas; samples were collected in southern and central areas. Volcanic rocks from the Deleni well are represented by basalts and basaltic andesites, which display a clear calc-alkaline affinity. Incompatible elements normalised to N-MORB exhibit typical Ta, Nb, and Ti negative anomalies, whereas chondrite-normalised REE patterns display decreasing trends from La to Lu. The relatively low contents of TiO2, Zr, Y, and Ti/V ratios (15-20), as well as the high Th/Yb ratios associated with relatively low Ta/Yb ratios suggest a formation in a supra-subduction setting. These rocks have chemical compositions very similar to those of equivalent island-arc calk-alkaline rocks from the SAM. Volcanic rocks from the Olt Nappe are represented by basalts, basaltic andesites, and trachytes, which display a clear alkaline affinity with Ba, Th, Ta, Nb, and LREE enrichments. Incompatible elements (including REE) abundances, Ti/V ratios (32-89) and high Th/Yb ratios associated with high Ta/Yb ratios are very similar to those observed in typical ocean-island basalts, and point out for a genesis in a within plate tectonic setting. As already suggested by, although no ophiolitic rocks were recovered in the Deleni-6042 deep well, the occurrence, at a depth of about 5000m, of calc-alkaline volcanics very similar to those associated with ophiolites in the SAM, indicate that the SAM ophiolitic nappe extend below the Transylvanian Depression for about 70km eastward. By contrast, alkaline within plate volcanics from the Olt Nappe have no equivalents in the SAM, thus suggesting that the Olt Nappe does not represent the easternmost end of the SAM ophiolitic nappe.