The Late Paleozoic Navarrete Complex, NE North Patagonian Massif, Argentina: geology, chemical characterization and magnetic fabric study

LOPEZ DE LUCHI, MÓNICA; RAPALINI, AUGUSTO E.; TOMEZZOLI, RENATA

Stellenbosch, South Africa

Simposio; Sixth Hutton Symposium on the Origin of Granites and related rocks; 2007

Geological Society of South Africa

The Late Paleozoic Navarrete Plutonic Complex (NPC, NE North Patagonian Massif) is a composite batholith which intruded the upper crustal rocks of the Nahuel Niyeu Fm (NNF) Highly deformed granitoids and orthogneisses of the Late Paleozoic Yaminue Complex (YC) appear at the western margin. As several areas are intruded by younger epizonal granites or covered by younger volcanic rocks the outcrop pattern of the batholith is difficult to depict. Where observed, contacts are NNE-SSW or N-S. trending. Plutons emplaced in NNF are NNE elongated and vary from 5- 20 km in length. Three main domains were depicted by field observations, petrography and the AMS study. Domains 1 and 2 are paramagnetic whereas domain 3 is ferromagnetic. Rocks are biotite coarse grained locally porphyric monzogranites (BCM), biotite ±amphibole coarse grained tonalites (BACT), amphibole biotite- porphyric tonalites (ABPT), biotite-amphibole porphyric granodiorites (BAPG), coarse grained granodiorites (BACG) and porphyric monzogranites (BAPM). Accesory minerals include apatite, zircon and sphene, particularly in the amphibole rich types. Stocks of amphibole-biotite diorites are observed. N-S trending fine grained diorite dykes are abundant at the western part of the batholith. Domain 1 (BCM) located to the northwest is characterized by a NW-SE planar fabric moderately dipping to the NE with moderate plunging NNE lineations. Domain 2 (BACT) occupies the central part of the outcrops and shows a steep NNW-NS planar fabric and moderate to steep N-S lineations. Domain 3 encompasses the only rocks that are in contact with the NNF. Subdomain 3a exhibits N-S trending planar fabric with either shallow to moderate E dipping and moderate to shallow ENE plunging lineations (BAPM) or moderate W dipping with moderate NW plunging lineations (BACG). Subdomain b (ABPT and BAPG) exhibits ENE planar fabric and shallow ENE-WSW plunging lineations. Subdomain c (BACG) is located at the NE sector of the Complex, the Puesto Navarrete area, and is characterized by moderate dipping foliations and mostly shallowly plunging lineations that are parallel to the contact with the NNF. Microstructures are mostly magmatic to submagmatic with a high-T subsolidus deformation in Domain 1. In the rest of the domains, a low-T subsolidus overprint is associated with localized NNE trending shear zones in some areas of the Domains 2 and 3 or ENE trending shear zones that control the development of a penetrative mylonitic fabric in Domain 1. Chemical information at present only corresponds to Domain 3. Rocks are metaluminous to weakly peraluminous. Most rocks define a high-K calk-alkaline series except for some alcali-calcic series fine grained diorites. The BACG belong to the medium K series and are characterized by Na2O/K2O > 1.8. Field and structural relationships indicate mixing-mingling process in most tonalites and granodiorites. High Ba, Sr, LREE may indicate a subduction related enrichment in the less evolved rocks (ABPT) whereas crustal sources are suggested by the rest of the granitoids. A comparison of the internal fabric of the granitoids with the structural elements in the NNF and the YC indicates that: -Domain 1 planar fabric is parallel to the D1 fabric in the northern part of the YC being both parallel to axial plane of the D2 folds of the NNF. At the time of Domain 1 rocks emplacement most YC would have been fully crystallized since rocks are granitic orthogneisses or alternatively they might represent areas of higher strain rate. Down dip lineations suggest top to the south west direction of movement under a regional NE-SW compression -Domain 2 planar fabric is parallel either to axial planes of the D3 folds of the NNF or to the D2 folds of the YC. Along strike steep lineations may indicate feeder zones -Sub domain 3a planar fabric is parallel to that of the Domain 2 but the associated lineations trend ENE. Fabrics in Domain 2 and Subdomain 3a suggest a roughly E-W compression. - ENE trending planar fabric and parallel lineations in Subdomain 3b may suggest a final stage of NW-SE compression. The published age of 282+-3 Ma, for the NPC corresponds to subdomain 3c rocks which show a pattern of internal deformation decoupled from the regional field. The Late Paleozoic NPC is mostly syn-kinematic. Domain 1 would have shared with the YC a partially common deformational history, most likely at the final stages of YC deformation. It is interpreted that NPC records the transition from a Late Paleozoic compressive event towards a period of quiescence or perhaps extension. The event of compression is bracketed by the U-Pb ages of ca 300 Ma for the YC and of 282+-3 Ma for the BACG of the Navarrete area. The regional NE-SW compressional event, that we found recorded by Domain 1, has been considered by some authors as related  to a frontal  collision of Patagonia with SW Gondwana.