Petrology and P-T evolution of garnet peridotites from central Sulawesi, Indonesia

Alpine-type orogenic garnet-bearing peridotites, associated with quartzo-fe ldspathic gneisses of a 140-115 Ma high-pressure/ultra-high-pressure metamo rphic (HP-UHPM) terrane, occur in two regions of the Indonesian island of S ulawesi. Both exposures are located within NW-SE-trending strike-slip fault zones. Garnet Iherzolite occurs as <10 m wide fault slices juxtaposed agai nst Miocene granite in the left-lateral Palu-Koro (P-K) fault valley, and a s 10-30 m wide, fault-bounded outcrops juxtaposed against gabbros and perid otites of the East Sulawesi ophiolite within the right-lateral Ampana fault in the Bongka river (BR) valley. Six evolutionary stages of recrystallizat ion can be recognized in the peridotites from both localities. Stage I, the precursor spinel Iherzolite assemblage, is characterized by Ol+Cpx+Opx+/-P rg-Amp+/-Spl+/-Rt+/-Phl, as inclusions within garnet cores. Stage II, the m ain garnet Iherzolite assemblage, consists of coarse-grained Ol +Opx+Cpx+Gr t; whereas finer-grained, neoblastic Ol+Opx+Grt+Cpx+/-Spl+/-Prg-Amp+/-Phl c onstitutes stage III. Stages IV and V are manifest as kelyphites of fibrous Opx+Cpx+Spl in inner coronas, and Opx+Spl+Prg-Amp+/-Ep in outer coronas ar ound garnet, respectively. The final (greenschist facies) retrogressive sta ge VI is accompanied by recrystallization of Serp+Chi +/-Mag+/-Tr+/-Ni sulp hides+/-Tlc+/-Cal. P-Tconditions of the hydrated precursor spinel Iherzolit e stage I were probably about 750 degrees C at 15-20 kbar. P-T determinatio ns of the peak stage IIc (from core compositions) display considerable vari ation for samples derived from different outcrops, with clustering at 26-38 kbar, 1025-1210 degrees C (P-K & BR); 19-21 kbar, 1070-1090 degrees C (P-K ), and 40-48 kbar, 1205-1290 degrees C (BR). Stage IIr (derived from rim co mpositions) generally records decompression of around 4-12 kbar accompanied by cooling of 50-240 degrees C from the IIc peak stage. Stage III, which p ost-dates a phase of ductile deformation, yielded 22+/-2 kbar at 750+/-25 d egrees C (P-K) and 16+/-2 kbar at 730+/-40 degrees C (BR). The granulite-am phibolite-greenschist decompression sequence reflects uplift to upper crust al levels from conditions of 647-862 degrees C at P = 15 kbar (stage IV), t hrough 580=635 degrees C at P = 10-12 kbar (stage V) to 350-400 degrees C a t P = 4-7 kbar (stage VI), respectively, and is identical to the sequence r ecorded in associated granulite, gneiss and eclogite. Sulawesi garnet perid otites are interpreted to represent minor components of the extensive HP-UH P (peak P > 28 kbar, peak T of c. 760 degrees C) metamorphic basement terra ne, which was recrystallized and uplifted in a N-dipping continental collis ion zone at the southern Sundaland margin in the mid-Cretaceous. The low-T, low-P and metasomatized spinel Iherzolite precursor to the garnet Iherzoli te probably represents mantle wedge rocks that were dragged down parallel t o the slab-wedge interface in a subduction/collision zone by induced corner flow. Ductile tectonic incorporation into the underthrust continental crus t from various depths along the interface probably occurred during the exhu mation stage, and the garnet peridotites were subsequently uplifted within the HP-UHPM nappe, suffering a similar decompression history to that experi enced by the regional schists and gneisses. Final exhumation from upper cru stal levels was clearly facilitated by entrainment in Neogene granitic plut ons, and/or Oligocene trans-tension in deep-seated strike-slip fault zones.