Paleomagnetic and Geochronologic Investigation of the Dzabkhan Microcontinent during the Neoproterozoic and Implications for the Assembly of Gondwana

A paleomagnetic and geochronologic investigation was undertaken on the Dzabkhan volcanics in the Altay region of southwestern Mongolia. Our goal is to define the paleolocation of the Dzabkhan microcontinent during the Neoproterozoic for the higher purpose of addressing the puzzling nature of pre-Phanerozoic glaciations and the tectonic evolution of the Ural-Mongol belt. A total of 184 samples (24 sites) were analyzed from a bimodal suite of volcanic rocks. Alternating field treatments up to 130 mT removed 90% of the natural remanent magnetization (NRM). Thermal demagnetization was applied up to temperatures of 565°C for the mafic samples and up to 680° C for some of the more felsic samples. Based on a preliminary analysis of the data, we identify 3 components of remanent magnetization in the Dzabkhan complex. Present day overprints unblocked at a temperature range of 100-250°C. Other components unblocked at higher temperature ranges of 530-550°C. The first component yielded a direction (in-situ) of 246.1/ -69.5, (k=86, a95=5). This component had a negative fold test. The second component had a shallower NW direction with in-situ mean 282.9/ -42.9 (k=109, a95=5.8). The sites were from a monocline and the tilt-corrected direction is 319/-75.4. The third component (also monoclinally dipping) yielded and in-situ mean of 306.4/24.7, (k=184, a95=5.7) and tilt corrected values 289.9/-59. All three components are high-temperature/high coercivity but a mean of the three components (either in-situ or tilt- corrected) is poorly grouped. A fold test conducted for components 1 and 3 is negative whereas a fold test for components 2 and 3 is positive above the 95% confidence interval. The overall mean for this tilt-corrected direction is 301.7/-69.1 (k=44.6, a95=6.6). This component is similar to, but statistically distinct from components observed in the overlying Tsagaan Oloom and Bayan Gol formations, but still may represent a remagnetization prior to folding. Intraformational conglomerates of the Dzabkhan volcanic clasts yield a low- temperature overprint with a mean direction of 267/-60.8 (similar to the post-folding direction observed in component #1) and random high temperature directions. The prefolding magnetization yielded a paleolatitude of ~53°. The Dzabkhan Formation underlies the diamictites at the base of the Tsagaan Oloom formation. We are in the process of determining an age of the felsic volcanics. Previous ages ranged from 850-700Ma. This new age will also provide some age constraints for the associated overlying diamictite and carbonates. We will discuss these directions and their significance to the paleoposition of the Dzabkhan microcontinent and the overall history of its amalgamation into the Eurasian continent.