7 Tectonic History of the Trans-Pecos Region

Thursday, 8 March 2012
1:30 PM - 5:00 PM
Espino A & B

LOWER CRUST XENOLITHS FROM TEL THANNOUN, SYRIA: INSIGHTS INTO THE LOWER CRUST OF AFRO-ARABIA (AND MAYBE TRANS-PECOS TEXAS)

Paper number: 7-1
Program: Theme Sessions
Time: 1:30 PM – 1:45 PM

Authors

Abstract

The nature of the lower crust (LC) beneath W Texas is poorly known so until we have more information from geophysics and xenoliths, we need to consider LC from crustal tracts of similar age and origin. Crust of NE Africa and Arabia was generated in Neoproterozoic time (~870-560 Ma), slightly younger than Mesoproterozoic (~1.3 Ga) crust of W Texas. We know a lot about exposed rocks of the Arabian-Nubian Shield (ANS), which is dominated by juvenile igneous rocks with mantle-like isotopic compositions. We know much less about ANS LC, which evolved as a buffer between the upper crust and upwelling mantle. Afro-Arabian LC has P-wave velocities indicating it is composed of mafic-intermediate igneous rocks. There are 8 localities in Arabia with young lava fields (harrats) where samples of ANS lower crust have been brought to the surface by late Neogene basanite eruptions. Here we report studies of eight xenoliths from one such locality (Tell Thannoun; TT) in southern Syria. Safarjalani et al. (2009, Chemie der Erde) studied 37 LC xenoliths from TT and nearby Tell Boutheina. These are dominated by mafic PL-OPX-CPX-BI-IL rocks, which Safarjalani et al. (2009) interpreted as metamorphosed marls and within-plate igneous rocks. In contrast, Krienitz & Haase (Chem Geo 2011) studied 5 LC samples from TT, concluding that these were pyroxene- and plagioclase-rich cumulates.

To address the controversy about the nature of ANS LC, we studied several TT xenoliths. The TT samples we examined are 2-pyroxene granulites of dominantly igneous origin, with 2-pyx equiibration T of 700-900°C. These samples represent a remarkable assemblage of deep lithospheric lithologies, from lherzolite (with Fo90) through pyroxenite, including the first example of sapphirine reported from NE Africa or Arabia, through 2-pyx gabbro to charnockite. Compositional diversity is revealed by range of orthopyroxene compositions (En45-En90). 2-pyx charnockite is particularly rich in zircon. Subordinate spinel, ilmenite, and perthitic feldspar is observed. We saw no evidence of metasedimentary protolith.These lithologies are generally consistent with lower crust velocities of 6.8 and 6.9 km/s beneath Jordan-Israel and SW Arabia, respectively, but the lithologic variability seen in this small sample suite implies remarkable complexity in the LC beneath TT.