Age and tectonic evolution of Neoproterozoic ductile shear zones in southwestern madagascar, with implications for gondwana studies

Abstract

Southern Madagascar comprises a complex Precambrian terrain of high-grade metamorphic rocks with a history of polyphase deformation and metamorphism. Two prominent N-S trending late Neoproterozoic ductile shear zones, the Ampanihy and Vorokafotra shears, each with projected strike length of > 450 km and between 10 and 20 km in width, crosscut the region. A third set of en echelon shears forms part of the early Paleozoic Ranotsara Shear Zone that cuts the basement in a NW-SE direction over a combined strike length of > 400 km. The host rocks of these shears comprise paragneisses (metasediments) with detrital zircons ranging in age between 720 and 1900 Ma. A felsic layer, interpreted as a metavolcanic rock, gives a date of 722±1 Ma. Remnants of late Archean orthogneisses in the central part of the study area may represent basement to the paragneisses. Four episodes of deformation and metamorphism have been recognized on the combined basis of field observations, petrogenesis, and U/Pb analyzes of zircons, monazites, sphenes, and rutiles. Two episodes of early simple shear deformation (D 1 and D 2) at midcrustal levels occurred between 627 and 647 Ma, during which northeast verging recumbent sheath folds and ductile thrusts were formed and peak prograde metamorphism reached 7-12 kbar at 750°-900°C. Early prolate mineral fabrics (L 1 /L 2) are preserved in massif-type anorthosite bodies and their marginal country rocks. D 1 occurred between 630 and 647 Ma, while D 2 occurred at 627-628 Ma. This was followed by a 10-15 Myr period of static, annealing metamorphism until 609-614 Ma when bulk shortening (D 3) took place. D 2 and D 3 are coaxial but are separated in time by leucocratic dykes that intruded between 610 and 620 Ma. D 3 was focused zonally, forming the prominent N-S shear zones between 607 and 609 Ma; its oblate strain resulted in a strong composite D 2 /D 3 fabric defined by subvertical S- tectonites and subhorizontal intersection lineations. A variety of post-D 3 pegmatites accompanied ∼85 Myr of relatively static annealing and metasomatic/metamorphic mineral growth, during which numerous occurrences of phlogopite, uranium, and rare earth elements formed. A continuum of concordant monazite dates suggests that this thermal event is part of an extended period of low-pressure (3-5 kbar) charnockite-producing processes between 520 and 605 Ma. The continuum, however, appears to be punctuated at ∼580, 550, and 520 Ma. Deformation (D 4) recorded within the Ranotsara Shear Zone overlaps with the youngest parts of the regional metamorphic conditions between 520 and 550 Ma. Prevailing low-pressure, high-temperature amphibolite-granulite facies rapidly gave way to greenschist facies conditions between 490 and 530 Ma. as is evident from overlapping ages of zircon, monazite, sphene, and rutile. We conclude that D 1 to D 3 represents a period of 40 Myr of compressional deformation that we interpret to be related to collisional events during the amalgamation of Gondwana. The first part of the thermal continuum between 550 and 605 Ma reflects ∼55 Myr of slow cooling and annealing at midcrustal levels, while the onset of the last episode, between 520 and 530 Ma, heralds accelerated exhumation accompanied by extensional tectonics between 490 and 520 Ma. We believe that this postcollisional time span represents a prolonged period of evolution of a Tibetan-style plateau into an Aegean-style extensional terrain. This ∼100 Myr event in southern Madagascar is similar to that recorded throughout large sectors of the East African Orogen between ca. 500 and 600 Ma. We believe that this type of postconvergent thermotectonism best represents the original definition of "Pan-African" [Kennedy, 1964], which in today's terminology equates with "postorogenic extensional collapse" [Dewey, 1988], or "destabilization of an orogen" [Lipps, 1998]. Kennedy's Pan-African was widespread throughout the interior a supercontinent, when Gondwana's periferal margins were subjected to far-field tensional forces. This suggests that neither gravitational collapse of the Pan-African-Braziliano Orogens nor delamination were the sole or even the dominant driving forces for the postconvergent extension.