INCEPTION AND DEMISE OF A NEOPROTEROZOIC OCEAN-BASIN - EVIDENCE FROM THE OUGDA COMPLEX, WESTERN HOGGAR (ALGERIA)

The Neoproterozoic Ougda magmatic complex occurs within platformal car bonate rocks in the western part of the Pan-African fold belt of the T uareg shield (NW Africa). It is composed of similar to 800 Ma old, rel atively high P-T (i.e., Grt + Cpx-bearing: P > 5 kbar; T similar to 90 0 degrees C), tholeiitic mafic/ultramafic cumulates and related rocks intruded by intermediate to mafic calc-alkali plutons (e.g., Cpx+Hbl-b earing gabbro) and dikes. Apparent contrasts in structural level of cr ystallization indicate that the calc-alkali rocks are significantly yo unger than the tholeiites, which temporally correlate with a period of regional extension in this part of Africa. Intrusion of the calc-alka li rocks may have occurred during the formation of an are after the th oleiitic rocks had been (diapirically?) emplaced within the shelf carb onates, and prior to (> 630 Ma) the Pan-African orogeny. Data reported herein indicate that the Ougda complex records the inception and demi se of a Neoproterozoic ocean basin. Similar crustal sections have been described from collisional (e.g., Aleutian islands) and extensional ( e.g., Ivrea-Verbano zone) settings, indicating that processes operatin g in both environments can generate nearly indistinguishable igneous s uites; the prevalence of shallow-level calc-alkali rocks in both setti ngs may mask the presence of more mafic, tholeiitic rocks at depth.