No abstract

We provide laser ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) and high-precision chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb ages and Hf isotopic compositions of zircons from the Red Bluff Granite Suite and mafic dikes in the Franklin Mountains of El Paso, Texas, USA. Granitoids exposed in the Franklin Mountains were previously divided into five magmatic stages based on cross-cutting relationships. Major and trace element compositions showed that these granitoids are ferroan, alkaline, and A2 type. Homogeneity in the whole-rock geochemistry suggests that the granite stages are genetically related and share similar petrogenetic histories. Weighted mean zircon 206Pb/238U dates from the older magmatic stage 1 alkali-feldspar quartz syenite and stage 2 alkali-feldspar granite are 1112.36 ± 0.35 and 1112.46 ± 0.37 Ma, respectively. The weighted mean εHf(t) values varying from +5.3 to +7.2 are similar to those of other regional ca. 1.1 Ga magmatic rocks throughout south- western Laurentia. Geochemical characteristics, petrological modeling, and enriched Hf isotopic composition suggest fractional crystallization of a basaltic magma that was produced by melting of an enriched mantle reservoir. However, zircon inheritance ages of ca. 1.3 Ga and 1.26–1.15 Ga are consistent with a minor contribution from felsic crustal basement. Our data and regional geology are consistent with a post-collisional slab break-off that facilitated asthenospheric upwelling and partial melting of the enriched mantle, possibly subcontinental lithospheric mantle, extending from Llano Uplift, Texas, in the southeast to California to the northwest. Magma thus generated upon differentiation produced ferroan and A-type granitoids.

is the oldest Precambrian rock unit (Stenian, ~1260Ma) exposed in the Franklin Mountains of West Texas. This unit is now marble, but was initially a carbonate/clastic sedimentary succession and has been metamorphosed to hornblende-hornfels facies. Originally named by Harbour (1960) as the "Castner Limestone," it contains exquisitely preserved bedding structures, including soft sediment deformation, imbricated edgewise conglomerates, and two types of stromatolites. Microbial Induced Sedimentary Structures (MISS) were first recognized in the Castner Formation by Pittenger (1994), who reported cryptalgal laminites. We describe several other MISS not associated with the previously described stromatolites. These MISS include gas domes, syneresis cracks, and possible discoidal microbial communities. In addition, we also offer an alternative hypothesis for the formation of edgewise conglomerates, namely that they may have formed due to microbial binding of individual beds, which has been reported elsewhere (i.e., Van Kranendonk et al., 2003).

Current fieldwork in the lowermost Albian-Cenomanian Mojado Formation (Sarten member) has produced several new invertebrate trace fossil discoveries. Reported here is the third documented occurrence of Planolites isp. in the Washita Group at Cerro de Cristo Rey Sunland Park, New Mexico, and the first record of this trace in the Mojado Formation. Planolites is a simple, sub-horizontal, cylindrical trace with fill that is different than the host rock. It is interpreted as a domichnia (dwelling trace) or fodichnia (feeding trace) with several possible tracemakers. It is preserved in most environmental settings including marine and continental, from the Ediacaran to Recent. At this locality this trace is associated with Arthrophycus, Thalassinoides, and Curvolithus, so we tentatively assign this ichnoassemblage to the Cruziana ichnofacies. Although ichnoassemblages attributed to the Cruziana ichnofacies have been described a few meters below in the Mesilla Valley Formation, these differ from the low-diversity ichnoassemblage in the Mojado Formation. This change in ichnoassemblage may in the future be used to define the lower contact of the Mojado Formationat Cristo Rey.