PETROGENESIS OF LATE NEOPROTEROZOIC DIKES IN THE NORTHERN ARABIAN-NUBIAN SHIELD - IMPLICATIONS FOR THE ORIGIN OF A-TYPE GRANITES

The petrogenesis of late Proterozoic dikes from the flat and Amram mas sifs (southern Israel) is constrained by major and trace element geoch emistry, and is linked to formation of alkaline (A-type) granites. Thr ee swarms of dikes intruded the late Proterozoic crustal rocks between 600 and 540 Ma. The older dikes strike N-S and are calc-alkaline (CA suite). A younger suite strikes NE-SW and is tholeiitic (TH suite). Bo th suites include dikes of mafic to felsic compositions (49-77 wt% SiO 2). Two alkali-basaltic dikes which dissect the former suites strike N W-SE (AB suite). Mafic dikes of all suites show relatively high Ni, V, and Cr concentrations (up to 120, 198, and 407 ppm, respectively) and high Mg# (up to 0.68). They display moderate LREE enrichment, lack Eu anomaly, and have flat HREE patterns. Nb is significantly lower in th e CA dikes compared to the concentrations in the tholeiitic and alkali -basaltic suites. Thermodynamical and mass balance modeling of major a nd trace elements strongly suggest that the range of lithologies in ea ch suite is related by crystal fractionation. Although the role of ass imilation is not significant, mixing by young crustal rocks or mixing over limited range of compositions cannot be ruled out. The CA dikes e volved by fractional crystallization of mainly amphibole, plagioclase, biotite, spinel, and apatite from hydrous andesitic magmas. Plagiocla se, alkali-feldspar and quartz crystallized from the dacitic and rhyol itic magmas. The tholeiitic dikes evolved by fractional crystallizatio n of anhydrous basaltic magmas by removal of clinopyroxene, plagioclas e, spinel, alkali-feldspar, quartz, apatite, and Fe-oxides. Rhyolites from the study area have the characteristics of A-type granites, with compositions that fall along the trends of the felsic tholeiitic dikes , suggesting co-genetic relations. At least some A-type felsic magmas in the Arabian-Nubian Shield are anhydrous fractionation products of b asaltic magmas. (C) 1998 Publisher by Elsevier Science B.V.