Eclogite to HP-granulite facies metamorphism in mafic rocks at Sierra de Umango, Argentina: relics of subducted ophiolite complex in western Gondwana?

GONZÁLEZ, PABLO D.; VARELA, RICARDO; VLACH, SILVIO

Mendoza

Simposio; Gondwana 12: Geological and Biological Heritage of Gondwana; 2005

Academia Nacional de Ciencias

Eclogite and granulite facies assemblages were described by Varela et al. (1996) within the Juchi Orthogneiss in the southernmost part of Sierra de Umango. In the northwestern part of this sierra, at Cerro de Cacho (28º51’S, 68º40’W), new outcrops of eclogite and high-pressure (HP) granulite facies assemblages have been discovered within mafic metamorphic rocks of the Tambillo Metamorphites. This unit is mainly composed of felsic gneisses, amphibolites and marbles, with minor schists, meta-gabbros and meta-ultramafic rocks. Relics of a retro-eclogite (terminology of HP rocks after Desmons and Smulikowski, 2004) containing eclogite and HP-granulite facies assemblages appear as inclusions in amphibolites. Mineral and textural analyses of all rock types and EPMAgeothermobarometry on minerals from retro-eclogite, together with published geochronological data (synthesis in Varela et al., 2003 and references therein) suggest that rocks of Tambillo Metamorphites follow a clockwise P-Ttime path, with at least four metamorphic events: (1) Relict MX-1 eclogite facies metamorphism characterized by plagioclase-free granoblastic garnet (alm54–59-pyr19–23-gros19–22-spe01–02) + diopside [(Wo+En+Fe)89–96-Jd02–05-Ae00–06)] + ilmenite + rutile. (2) Peak metamorphic conditions at MX-2 HP-granulite facies (P-T range ≈12-14 kb and 731–853°C), with complex coronitic, symplectitic and kelyphitic reaction textures of amphibole (ferropargasite to Fe-Mg hornblende) + Ab-rich plagioclase [oligoclase (Ab70–75) to andesine (Ab61–69)] around MX-1 garnet-diopside assemblage. MX-1 and MX-2 assemblages are only preserved in a retro-eclogite amphibolite. (3) Retrograde MX-3 amphibolite facies metamorphism (U–Pb age: 452–447 Ma) characterized by nematoblastic hornblende + plagioclase + sphene (corona around ilmenite) ± garnet ± quartz in amphibolites (T range of 630–694°C at estimated 5 kb), and quartz + plagioclase + microcline + garnet + biotite1 in felsic gneisses. The Late Ordovician age of MX-3 event can be ascribed to the early stages of the late- to post-orogenic phase of Famatinian orogeny of the Sierras Pampeanas. Thus, the peak MX-2 event might be associated with the Early Ordovician main orogenic phase of the Famatinian orogeny, but we can not rule out metamorphism ages older than Famatinian for MX-1-MX-2 events. (4) MX-4 greenschist facies shear zone metamorphism (Rb–Sr, K–Ar ages: 390–370 Ma) associated with mylonitic foliation formation and accompanied by C-type shear bands marked by biotite2 + chlorite alignment. Within the clockwise P-T-t path, the Tambillo Metamorphites entered the P-T field of the MX-2 HP-granulite facies (experiencing isobaric heating or heating with decompression) via the MX-1 eclogite facies. These rocks were later exhumed and cooled to crustal levels equivalent to MX-3 amphibolite and MX-4 greenschist facies conditions. This type of trajectory is associated with collisional belts in which parts of ophiolitic complexes are subducted into the eclogite field, and afterwards experience HP-granulite facies conditions prior to slow exhumation and cooling (O’Brien and Rötzler, 2003). Within the geodynamic scenario of the Famatinian subduction zone along the western margin of Gondwana during the Ordovician, the Tambillo Metamorphites might represent disrupted parts of an ophiolitic complex (metasediments + metamorphosed mafic–ultramafic rocks) subducted beneath the autochthonous continental crust. This complex was exhumed after the collision of a Grenvillian basement with the autochthon border, during the orogenic collapse. Thus, the strips of Tambillo Metamorphites from Cerro de Cacho possibly lie within and along the suture zone, where the highest pressure and temperature related to the collision are recorded. Within the suture zone, the rocks belonging to the allochthonous terrane, Gondwana margin and exhumed ophiolitic complex are tectonically juxtaposed at the same crustal level. Taking into account that the protoliths of Tambillo Metamorphites are possible parts of an oceanic crust consumed during closure of an ocean between two continental blocks, they neither form part of the allochthonous terrane nor belong to the Gondwana margin.HP) granulite facies assemblages have been discovered within mafic metamorphic rocks of the Tambillo Metamorphites. This unit is mainly composed of felsic gneisses, amphibolites and marbles, with minor schists, meta-gabbros and meta-ultramafic rocks. Relics of a retro-eclogite (terminology of HP rocks after Desmons and Smulikowski, 2004) containing eclogite and HP-granulite facies assemblages appear as inclusions in amphibolites. Mineral and textural analyses of all rock types and EPMAgeothermobarometry on minerals from retro-eclogite, together with published geochronological data (synthesis in Varela et al., 2003 and references therein) suggest that rocks of Tambillo Metamorphites follow a clockwise P-Ttime path, with at least four metamorphic events: (1) Relict MX-1 eclogite facies metamorphism characterized by plagioclase-free granoblastic garnet (alm54–59-pyr19–23-gros19–22-spe01–02) + diopside [(Wo+En+Fe)89–96-Jd02–05-Ae00–06)] + ilmenite + rutile. (2) Peak metamorphic conditions at MX-2 HP-granulite facies (P-T range ≈12-14 kb and 731–853°C), with complex coronitic, symplectitic and kelyphitic reaction textures of amphibole (ferropargasite to Fe-Mg hornblende) + Ab-rich plagioclase [oligoclase (Ab70–75) to andesine (Ab61–69)] around MX-1 garnet-diopside assemblage. MX-1 and MX-2 assemblages are only preserved in a retro-eclogite amphibolite. (3) Retrograde MX-3 amphibolite facies metamorphism (U–Pb age: 452–447 Ma) characterized by nematoblastic hornblende + plagioclase + sphene (corona around ilmenite) ± garnet ± quartz in amphibolites (T range of 630–694°C at estimated 5 kb), and quartz + plagioclase + microcline + garnet + biotite1 in felsic gneisses. The Late Ordovician age of MX-3 event can be ascribed to the early stages of the late- to post-orogenic phase of Famatinian orogeny of the Sierras Pampeanas. Thus, the peak MX-2 event might be associated with the Early Ordovician main orogenic phase of the Famatinian orogeny, but we can not rule out metamorphism ages older than Famatinian for MX-1-MX-2 events. (4) MX-4 greenschist facies shear zone metamorphism (Rb–Sr, K–Ar ages: 390–370 Ma) associated with mylonitic foliation formation and accompanied by C-type shear bands marked by biotite2 + chlorite alignment. Within the clockwise P-T-t path, the Tambillo Metamorphites entered the P-T field of the MX-2 HP-granulite facies (experiencing isobaric heating or heating with decompression) via the MX-1 eclogite facies. These rocks were later exhumed and cooled to crustal levels equivalent to MX-3 amphibolite and MX-4 greenschist facies conditions. This type of trajectory is associated with collisional belts in which parts of ophiolitic complexes are subducted into the eclogite field, and afterwards experience HP-granulite facies conditions prior to slow exhumation and cooling (O’Brien and Rötzler, 2003). Within the geodynamic scenario of the Famatinian subduction zone along the western margin of Gondwana during the Ordovician, the Tambillo Metamorphites might represent disrupted parts of an ophiolitic complex (metasediments + metamorphosed mafic–ultramafic rocks) subducted beneath the autochthonous continental crust. This complex was exhumed after the collision of a Grenvillian basement with the autochthon border, during the orogenic collapse. Thus, the strips of Tambillo Metamorphites from Cerro de Cacho possibly lie within and along the suture zone, where the highest pressure and temperature related to the collision are recorded. Within the suture zone, the rocks belonging to the allochthonous terrane, Gondwana margin and exhumed ophiolitic complex are tectonically juxtaposed at the same crustal level. Taking into account that the protoliths of Tambillo Metamorphites are possible parts of an oceanic crust consumed during closure of an ocean between two continental blocks, they neither form part of the allochthonous terrane nor belong to the Gondwana margin.