Comparative lithological and geochemical studies of the Palaeozoic rocks from western Bulgaria

Abstract

First attempt to link the lithological data for Ordovician, Silurian, Devonian and Upper Carboniferous rocks from the Svoge Anticlinorium and the Sofiyska Stara Planina Mountain to the results from the multivariate mathematics analysis of data for macro- and microchemical composition of the rocks (correspondence and cluster analysis) was made in the present study. Four groups with enhanced content of the certain macroelements were divided: I - with SiO2; II - with CaO; III - with Na2О; III - with Al, Fe, Mn, Mg, K, volatile and Ti; and 9 subgroups (Ia, Ib, Ic, IIIa, IIIb, IIIc, IVa, IVb, IVc). The group Ia includes Ordovician quartzites, Upper Devonian sandstones, Devonian silicites and Upper Carboniferous sandstones; Ib is composed of mainly Upper Carboniferous polymictic sandstones; Ic consists of argillites of Ordovician, Silurian and Devonian ages. Group II contains sandstones abundant in Ca. Group III comprises sandstones of various ages and with diverse degree of transformation (catagenetic and low metamorphic) or with assistance of volcanic rocks in the source province. Group IV contains argillites: IVa - from the Ordovician Grohoten Formation, tightly linked to Al and K; IVb - from the Tzeretzel Formation, from Upper Silurian, Lower and Middle Devonian, and a part of the Upper Carboniferous argillites; IVc - argillites from the transitional interval between Silurian and Devonian as well as chert argillites from the Lower Silurian. Four groups were determined on the dendrogram obtained after cluster analysis with respect to the microcomposition. The first group (I) contains samples with high amount of Mn (Tzeretzel Formation, Upper Silurian, Middle and Upper Devonian silicites and argillites, Carboniferous sandstones and argillites). The second group (II) is abundant of V, Cr, Rb (Silurian graptolite argillites, less Devonian, Ordovician and Upper Carboniferous argillites). The group III is abundant of Zr (clastic rocks from the Grohoten Formation and from Upper Carboniferous) and group IV is composed of samples with high Sr content (mainly Upper Devonian). The detached groups regarding to macro- and microcomposition are resulting from mineralogical and petrographical features of the rocks and thus they correlate with each other in a certain degree. A certain tendency of variations in the geochemical features of the rocks were established regarding to their evolution from mature sediments (Ordovician and Silurian) to immature sediments (Upper Devonian, Upper Carboniferous), to he change of marine sedimentation (Ordovician - Devonian) to continental one (Upper Carboniferous); to the change in the provenance (sedimentary rocks in the Ordovician and the Silurian, the same plus metamorphic in the Devonian, previous plus intrusivc rocks in the Upper Carboniferous); to the influence of the climate (weaker in the marine deposits and stronger in the continental Upper Carboniferous) etc.