The Zavkhan terrane is a Proterozoic cratonic fragment in southwestern Mongolia that forms the core of the Central Asian orogenic belt. We provide new geologic and U-Pb zircon geochronologic constraints on the Neoproterozoic and early Paleozoic tectonic evolution of the terrane. Orthogneisses dated as ca. 1967 and ca. 839 Ma form the basement and are intruded and overlain by ca. 811-787 Ma arc-volcanic and volcaniclastic rocks that lack a gneissic fabric, suggestive of a mid-Neoproterozoic metamorphic event. Rifting and formation of the Zavkhan ribbon continent occurred from ca. 770-717 Ma and was followed by passive margin sedimentation between 717 and 580 Ma. During the latest Ediacaran to Cambrian, the southern margin of the Zavkhan terrane was reactivated with the obduction of the Lake terrane, slab break-off and reversal, and ca. 509-507 Ma magmatism. Metamorphosed Proterozoic and Cambrian units are cut by undeformed ca. 496 Ma gabbro, providing a tight constraint on the age of Cambrian metamorphism. Late Ordovician to Silurian rifting is marked by bimodal magmatism and deposition in narrow fault-bound basins. Our data indicate that the Zavkhan terrane traveled alone in the Neoproterozoic, collided with the Lake terrane in the late Ediacaran to Cambrian, accreted an unknown crustal block during Cambrian Epoch 2-Epoch 3, and then rifted away in the Ordovician. We suggest the majority of continental growth in Mongolia occurred through the trapping and oroclinal bending of ribbon continents rather than long-lived accretion on the margin of a major craton.

Geochemistry of high-K basalts and hydrous experiments saturated with biotite and/or amphibole in comparison to the Dariv high Mg# (>60) dike compositions. (a) K2O/Na2O v. SiO2 (note that x-axis is a logarithmic scale, for a-e), (b) Al2O3 v. SiO2, (c) TiO2 v. SiO2, (d) CaO-NaO-K2O. Mexican Volcanic Belt high Mg# alkali-basalts and lamprophyre data (colored circles

Quantifying the time scales of magmatic differentiation is critical for understanding the rate at which silicic plutonic and volcanic rocks form. Directly dating this process is difficult because locations with both clear evidence for fractional crystallization and the accessory phases necessary for radiometric dating are rare. Early zircon saturation, however, appears to be characteristic of many high-K, arc-related melts due to their generally elevated initial Zr concentrations. Thus, high-K plutonic series are ideal candidates to study the time scales of magmatic differentiation using zircon U-Pb geochronology. This study focuses on the Dariv Igneous Complex in western Mongolia where early saturation of zircon in a suite of cogenetic, upper crustal (<0.5 GPa) igneous rocks ranging from ultramafic cumulates to evolved granitoids allows us to date magmatic differentiation. Crystallization ages from six samples across the sequence indicate that magmatic fractionation from a basalt to high-silica (>65 wt% SiO2) melt occurred in ≤590 ± 350 k.y. This estimate is greater than modeled time scales of conductive cooling of a single intrusion and physical segregation of minerals from a melt, suggesting that continued influx of heat through magmatic activity in the complex may have prolonged cooling and thus time scales associated with the production of silica-enriched melts.

Phlogopite-and clinopyroxene-dominated fractional crystallization of an alkaline primitive melt: petrology and mineral chemistry of the Dariv Igneous Complex, Western Mongolia The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Bucholz, Claire E., Oliver Jagoutz, Max W. Schmidt, and Oyungerel Sambuu. "Phlogopite-and clinopyroxene-dominated fractional crystallization of an alkaline primitive melt: petrology and mineral chemistry of the Dariv Igneous Complex, Western Mongolia." Contributions to Mineralogy and Petrology 167:4 (2014 Mar 27):article 994.

Determining what caused the global Last Glaciation and last glacial termination, despite opposing orbital summer insolation signatures between the polar hemispheres, remains a puzzle of paleoclimatology. This problem can be addressed by comparing chronologies of glaciation from different latitudes and different climatic regimes in both hemispheres. Here, we present a 10Be surface‐exposure chronology of glacial landforms constructed during and since the local Last Glaciation in the continental environment of Central Asia in the high Mongolian Altai (49°N, 88°E). Four belts of lateral moraines document maximal phases of the former Khoton glacier at 35,440 ± 980 years ago, 23,430 ± 850 years ago, 20,780 ± 610 years ago, and 19,520 ± 550 years ago. Our chronology indicates that deglaciation from these maximal positions began as early as 18,810 ± 510 years ago, was well underway by 17,680 ± 510 years ago, and was nearly completed by 16,040 ± 490 years ago. Overall, our chronology shows that glaciation in western Mongolia overlapped with the global Last Glacial Maximum and that extensive recession from glacial‐to‐interglacial limits took place early in the last glacial termination during Heinrich Stadial 1. Khoton Nuur deglaciation led the demise of large Northern Hemisphere ice sheets and increases in radiative forcing agents by several millennia. We suggest that this rapid switch in the mode of glaciation implies the involvement of an additional climatic factor that could have produced locally rapid warming and deglaciation ∼18,800–16,000 years ago.