P-T-TIME CONSTRAINTS ON THE METAMORPHIC ROCKS OF NORTH MADAGASCAR AND THEIR RELEVANCE ON THE ASSEMBLY OF GONDWANALAND

U-Pb geochronology, metamorphic petrology, and P-T calculations of Precambrian rocks in north Madagascar provide essential constraints upon the continental configuration of Rodinia and models of Gondwana's amalgamation. The Precambrian rocks of northern and central Madagascar may be broadly divided into three tectonic elements: (1) a northern, largely juvenile Neoproterozoic volcanic/plutonic arc terrane; (2) a southern, stable shield consisting of late Archean granitoids and supracrustal gneisses, and between terranes 1 and 2, a zone of high-grade metamorphic rocks (3=Sambarino Group and the basal Tsarantanana thrust zone) including orthopyroxene-bearing granitoids (charnockites) emplaced into both the southern, Archean shield (2) the Sambarino Group (3). Terrane (1) consists of ~750-714 Ma plutonic and volcanic igneous rocks with trace element and Nd isotopic compositions (gNd (T)=+2.4-2.8) suggestive of rapid derivation from depleted mantle sources. The Neoproterozoic igneous rocks of North Madagascar are strikingly similar in age, chemistry, and isotopic characteristics to the igneous rocks of the Seychelles and Rajasthan (India), and hence an Andean-type arc origin has been proposed for all of them. The Sambirano Group (3) consists of metaconglomerate, graywacke, and pelite, as well as lesser orthoquartzite and calc-silicate gneiss, variably metamorphosed to greenschist grade (in the northeast) and amphibolite grade (in the southwest) constrained by garnet-cordierite thermobarometry to peak conditions of ~850 °C and ~7 kbar. Similar temperatures and pressures were obtained from stromatic pelitic migmatite using GASP barometry. Based on U-Pb geochronology of monazite in crd-gar-kspar-sill gneiss (~520 Ma), zircon in charnockite (~521 Ma), and recrystallized sphene (~511 Ma) in terrane 1, peak metamorphism occurred in ~Middle Cambrian time. Thus the Sambirano Group and related charnockites, are interpreted to be part of a larger Cambrian-age orogenic belt bounded on the south by the Tsarantana thrust zone that separates a juvenile volcanic/plutonic arc terrane (North Madagascar) from an Archean craton (Central Madagascar) of unknown Rodinian affinity (East or West Gondwanan, or neither). The possible extension of this orogenic belt into Somalia and India will be discussed.