DETRITAL ZIRCON GEOCHRONOLOGY OF SIERRA DE MAZ: IMPLICATIONS FOR PROVENANCE OF THE WESTERN MARGIN OF GONDWANA

The Sierra de Maz range of northwestern Argentina lies in the MARA block, composed of terranes transferred from Laurentia to Gondwana in the Neoproterozoic-early Paleozoic. The timing of MARA accretion to Gondwana, and the location of the boundary, is unknown. I present U-Pb dates of detrital zircon from four quartzite samples: two samples in north-central Sierra de Maz, and two samples from approximately 5 km west. These data provide insight to the maximum depositional age of the metasediment and the various provenance sources.

All samples contain small populations of Neoproterozoic grains as well as dominant peaks of 1.0 -1.2 Ga grains that make up 50-90% of the sample populations. A sample in north-central Sierra de Maz contains minor peaks at 1.28 and 1.36 Ga and various single grains that span from 595-819 Ma. Slightly to the south, another sample shows minor peaks at 1.34, 1.44, and 1.64 Ga, and single grains spanning from 576-823 Ma. The two samples to the west show very similar distributions with major peaks around 1.85 Ga, minor peaks around 2.5-2.65 Ga and single grains around 680 Ma. One of the samples contains a 2.84 Ga peak and various single grains between 2.1 and 2.3 Ga.

Proterozoic orthogneiss crystallized at 1.0-1.2 Ga is the dominant igneous rock east of these metasediments, separated from them by a Devonian sinistral shear zone. Sources for the Paleoproterozoic zircons are not exposed in the greater region. The 1.0-1.2 Ga event is very common throughout the western margin of Argentina, and our samples display similarities and differences to detrital zircon data in comparable units to the north and south of Sierra de Maz. To the south, near Pie de Palo, major detrital zircon signatures of roughly 1.3-1.47 Ga closely resemble the populations seen in our samples and are interpreted as being deposited on Laurentia. However, these regions lack the presence of the young (ca. 500-600 Ma) and older populations (2.5-2.7 Ga) seen in our data. The Neoproterozoic Puncoviscana Formation, deposited on Gondwana, is dominated by younger grains (500-650 Ma), but the samples lack the dominant 1.0-1.2 Ga peak and Paleoproterozoic grains seen in our data. Our data suggests that this metasediment was deposited on MARA and experienced a prolonged depositional history and/or has a wide variety of crystalline basement rocks as source areas.