A B S TRACT: X-ray diffraction chlorite crystallinity (ChC) indices and major element chemical compositions of chlorites and bulk rocks were determined and correlated in meta-igneous rocks from different Mesozoic formations in various tectonic units of the Biikk Mountains, NE Hungary. The rocks, of basic to acidic compositions, range from ocean-floor metamorphic prehnite-pumpellyite facies (diagenetic zone) through regional metamorphic prehnite-pumpellyite facies (anchizone) up to the regional metamorphic pumpellyite-actinolite and greenschist facies (epizone). As in the case of meta-sedimentary rocks, chlorite crystallinity can be applied as an empirical, complementary petrogenetic tool to determine relative differences in grades of low-temperature meta-igneous rocks. Electron microprobe and XRD data show that ChC is controlled mainly by the decreasing amounts of contaminants (mixed-layered components or discrete, intergrown phases of mostly smectitic composition) in chlorite with advancing metamorphic grade, up to the epizone. The apparent increase in calculated AI iv content-0f chlorite with increasing temperature is related to the decrease of these contaminants, as stated earlier by . On the basis of the significant correlations found between ChC and temperatures, derived by the chlorite-AP v geothermometer of Cathelineau (1988), both methods may be used for estimating the approximate temperatures of metamorphism, in spite of the contrasting interpretation of chemical data from chlorites obtained by electron microprobe analyses. After determining the effects of changing bulk chemistry on chlorite composition and ChC, the chlorite crystallinity method may complement the correlation of the illite crystallinity-based zonal classification of meta-sediments and the mineral facies classification of meta-igneous rocks.Determination of grade at subgreenschist facies has utilized a variety of approaches ranging from metamorphic facies to the illite crystallinity zonal scheme. The illite crystallinity technique (Ktibler, 1967(Ktibler, , 1968 has been widely applied to sequences with abundant pelitic rocks. For igneous rocks of basic-intermediate composition, a variety of facies has been defined (Liou et aL, 1987). Various attempts have been made to correlate these two approaches (for a review see Kisch, 1987). However, the occurrences of diagnostic mineral assemblages are strongly restricted, not only by the whole-rock chemistry but also by the composition of the fluid phase. Thus, in the case of CO2-containing fluids, non-diagnostic mineral assemblages consisting of chlorite, albite, quartz, carbonate minerals _+ white K-mica, titanite and opaque minerals occur commonly in basic-intermediate meta-igneous rocks. As these assemblages are stable over a relatively broad temperature range (from late diagenesis through the very low-grade realm up to the low-temperature part of the greenschist facies), they are inadequate for determination of metamorphic conditions.Recently, increased attention has been paid to the struc...

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The volcano-sedimentary succession around Wadi Zaghra in Sinai, at the northernmost segment of the Arabian Nubian Shield, comprises volcanic rocks interbedded with rather immature sediments. The succession is dominated by intermediate to silicic volcanics of medium-to high-K calc-alkaline affinity. It is divided into two units, the lower unit includes intermediate rocks and dacites interbedded with graywackes, semi-pelites and pelites and topped by polymict conglomerates. This unit is subjected to folding and regional metamorphism (up to garnet zone) and is intruded by quartz diorite-granodiorite inducing, locally, low-pressure contact thermal metamorphism. The unmetamorphosed upper unit encompasses acid volcanics intercalated with litharenite, sublitharenite and minor arenite. The rhyolites of this unit pertain to the highly fractionated granites and are characterized by an agpaitic index (NK/A) ranging from 0.87 to 0.96. They may reflect either extensive interaction of subduction-related magmas with the continental crust or a change in the tectonic regime. The present lithological and geochemical characteristics of the studied sediments together with available zircon ages indicate rather distal provenance of their detritus. This detritus comprises fluvial-alluvial sediments accumulated in the intermontane basins, which are half-grabens or tilted fault blocks. The tectonic setting of the depositional basins is active continental margin and continental island arcs. Geochemical patterns of the Zaghra volcano-sedimentary succession indicate their correlation with the Dokhan Volcanics-Hammamat Clastics sequence of the Eastern Desert of Egypt. Also, the Zaghra volcanics display geochemical similarities with those exposed in Sinai, at the Rutig, Ferani and Iqna Shar'a areas. The Zaghra succession is dated as Ediacaran but is not related either to the ensimatic island arc assemblage or to the rift-related assemblage formed during the early stages of the break-up of Rodinia as previously thought.