EARLY-MIOCENE POSTCOLLISIONAL CALC-ALKALINE MAGMATISM ALONG THE EASTERNMOST SEGMENT OF THE PERIADRIATIC FAULT SYSTEM (SLOVENIA AND CROATIA)

Early Miocene calc-alkaline igneous rocks from the easternmost segment of the Periadriatic fault system can be subdivided into two series of different alkalinity: (1) Volcanics which occur in the vicinity of th e Velenje-Rogatec Line range from high-alumina basalt via low-K and me dium-K andesites to medium-K dacites. (2) In the Pohorje Mountains maf ic rocks are lacking. A high-K tonalitic pluton crystallized at pressu res of about 0.7 GPa as indicated by Al-in-hornblende barometry and ac cessory magmatic epidote. Rapid exhumation of the tonalite during dext ral transtension along the Periadriatic fault system is indicated by t onalitic pebbles in Helvetian (?) elastic sediments. High-K andesitic to dacitic volcanics are interlayered with, and dikes cut, the elastic sediments. Compositional variations shown by the volcanics from the V elenje-Rogatec Line are consistent with fractionation of the observed phenocryst assemblages (olivine, plagioclase, clinopyroxene, orthopyro xene, titanomagnetite). Substantial fractionation of plagioclase is in dicated by decreasing Sr/Nd and Sr/Y values and increasing negative Eu anomalies with increasing SiO2. All samples have chondrite-normalized HREE >9.6 and low (Tb/Yb)(n) ratios (1.29-1.08). With increasing SiO2 , the abundances of HREE and Y (18 to 39 ppm) increase and those of Sc (32.5 to 20.9 ppm) decrease slightly. These features, together with l ow Hf/Lu and Zr/Y values (10.1-5.7 and 5.7-3.6, respectively), rule ou t garnet as a major fractionating phase. Since (La/Yb)(n) values (3.24 -6.59) do not increase with SiO2 and chondrite-normalized REE patterns do not show concave-upward shapes, fractionation of amphibole was pro bably insignificant. Although Rb/Cs values(less than or equal to 18) a re generally low, a significant contribution by continental crustal ma terials to the magmas by an AFC process is suggested by negative corre lations of Nb/U (7.16 to 3.14) and Nb/Th (2.14 to 0.87) values with Si O2, as well as by low Hf/Ta (7.8-3.4), and in part also by K/Rb (340-7 1) and Zr/Rb (5.0-1.7) values. Rocks from the Pohorje Mountains have h igh abundances of U. Values of Nb/U and Nb/Th are low (3.55 to 1.85 an d 1.27 to 0.64, respectively) and are negatively correlated with SiO2. These features, in combination with high values of Ba/Nb (56 to 93), Ba/La (23-30), and Rb/Cs (19-56), as well as with previously published delta(18)O values (similar to 9 parts per thousand) for the tonalite indicate a substantial contribution of the continental crust to these magmas. High Sr abundances (455 to 984 ppm) and moderate negative Eu a nomalies suggest that fractionation of plagioclase was of minor import ance. Although the rocks have relatively low Sc (9.3 to 3.8 ppm) and Y (21 to 14 ppm) contents, low (Tb/Yb)(n) values (1.67-1.14) indicate t hat garnet was not a major fractionating phase. Instead, fractionation of amphibole is a viable mechanism to explain the combination of high (La/Yb)(n) (21.8-13.1) and low (Tb/Yb)(n).