Abstract 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.
<p>The classical model for the collision between India and Eurasia, which resulted in the formation of the Himalayan orogeny, is a single-stage continent-continent collision event at around 55 &#8211; 50 Ma. However, it has also been proposed that the India-Eurasia collision was a multi-stage process involving an intra-oceanic Trans-Tethyan subduction zone south of the Eurasian margin. We present paleomagnetic data constraining the location the Kohistan-Ladakh arc, a remnant of this intra-oceanic subduction zone, to a paleolatitude of 8.1 &#177; 5.6 &#176;N between 66 &#8211; 62 Ma. Comparing this result with new paleomagnetic data from the Eurasian Karakoram terrane, and previous paleomagnetic reconstructions of the Lhasa terrane reveals that the Trans-Tethyan Subduction zone was situated 600 &#8211; 2,300 km south of the contemporaneous Eurasian margin at the same time as the first ophiolite obduction event onto the northern Indian margin. Our results confirm that the collision was a multistage process involving at least two subduction systems. Collision began with docking between India and the Trans-Tethyan subduction zone in the Late Cretaceous and Early Paleocene, followed by the India-Eurasia collision in the mid-Eocene. The final stage of India-Eurasia collision occurred along the Shyok-Tsangpo suture zone, rather than the Indus-Tsangpo. The addition of the Kshiroda oceanic plate, north of India after the Paleocene reconciles the amount of convergence between India and Eurasia with the observed shortening across the India&#8211;Eurasia collision system. Our results constrain the total post-collisional convergence accommodated by crustal deformation in the Himalaya to 1,350 &#8211; 2,150 km, and the north-south extent of the northwestern part of Greater India to < 900 km.</p>
Abstract During the differentiation of arc magmas, fractionating liquids follow a series of cotectics, where the co‐crystallization of multiple minerals control the melt compositional trajectories, commonly referred to as liquid lines of descent (LLD). These cotectics are sensitive to intensive properties, including fractionation pressure and melt H 2 O concentration, and changes in these variables produce systematic differences in the LLDs of arc lavas. Based on a compilation of experimental studies, we develop two major element proxies that exploit differences in LLDs to constrain the fractionation conditions of arc magmas. Near‐primary fractionating magmas evolve along the olivine‐clinopyroxene cotectic, which is pressure‐sensitive. We use this sensitivity to develop a proxy for early fractionation pressure based on the normative mineral compositions of melts with 8 ± 1 wt.% MgO. Fractionation in more evolved magmas is controlled by the clinopyroxene‐plagioclase cotectic, which is strongly sensitive to magmatic H 2 O contents. We use this relationship to develop an H 2 O proxy that is calibrated to the normative mineral components of melts with 2–4 wt.% MgO. These two proxies provide new tools for estimating the variations in pressure and temperature between magmatic systems. We applied these proxies to compiled major element data and phenocryst assemblages from modern volcanic arcs and show that in island arcs early fractionation is relatively shallow and magmas are dominantly H 2 O‐poor, while continental arcs are characterized by more hydrous and deeper early fractionation. These differences likely reflect variations in the relative contributions of decompression and flux melting in combination with distinct upper plate controls on arc melt generation.
Research Article| December 01, 2007 The rift-to-drift transition in the North Atlantic: A stuttering start of the MORB machine? Oliver Jagoutz; Oliver Jagoutz 1Institut für Geologie, Universität Bern, CH-3012 Bern, Switzerland Search for other works by this author on: GSW Google Scholar Othmar Müntener; Othmar Müntener 1Institut für Geologie, Universität Bern, CH-3012 Bern, Switzerland Search for other works by this author on: GSW Google Scholar Gianreto Manatschal; Gianreto Manatschal 2Centre géochimie surface-École et observatoire des sciences de la Terre, Université Louis Pasteur, F-67084 Strasbourg, France Search for other works by this author on: GSW Google Scholar Daniela Rubatto; Daniela Rubatto 3Research School of Earth Sciences, Australian National University, Canberra 0200 ACT, Australia Search for other works by this author on: GSW Google Scholar Gwenn Péron-Pinvidic; Gwenn Péron-Pinvidic 4Centre géochimie surface- École et observatoire des sciences de la Terre, Université Louis Pasteur, F-67084 Strasbourg, France Search for other works by this author on: GSW Google Scholar Brent D. Turrin; Brent D. Turrin 5Department of Geological Sciences, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA Search for other works by this author on: GSW Google Scholar Igor M. Villa Igor M. Villa 6Institut für Geologie, Universität Bern, CH-3012 Bern, Switzerland, and Universita di Milano Bicocca, 20126 Milano, Italy Search for other works by this author on: GSW Google Scholar Author and Article Information Oliver Jagoutz 1Institut für Geologie, Universität Bern, CH-3012 Bern, Switzerland Othmar Müntener 1Institut für Geologie, Universität Bern, CH-3012 Bern, Switzerland Gianreto Manatschal 2Centre géochimie surface-École et observatoire des sciences de la Terre, Université Louis Pasteur, F-67084 Strasbourg, France Daniela Rubatto 3Research School of Earth Sciences, Australian National University, Canberra 0200 ACT, Australia Gwenn Péron-Pinvidic 4Centre géochimie surface- École et observatoire des sciences de la Terre, Université Louis Pasteur, F-67084 Strasbourg, France Brent D. Turrin 5Department of Geological Sciences, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA Igor M. Villa 6Institut für Geologie, Universität Bern, CH-3012 Bern, Switzerland, and Universita di Milano Bicocca, 20126 Milano, Italy Publisher: Geological Society of America Received: 18 Dec 2006 Revision Received: 25 Jul 2007 Accepted: 27 Jul 2007 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 The Geological Society of America, Inc. Geology (2007) 35 (12): 1087–1090. https://doi.org/10.1130/G23613A.1 Article history Received: 18 Dec 2006 Revision Received: 25 Jul 2007 Accepted: 27 Jul 2007 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Oliver Jagoutz, Othmar Müntener, Gianreto Manatschal, Daniela Rubatto, Gwenn Péron-Pinvidic, Brent D. Turrin, Igor M. Villa; The rift-to-drift transition in the North Atlantic: A stuttering start of the MORB machine?. Geology 2007;; 35 (12): 1087–1090. doi: https://doi.org/10.1130/G23613A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We report U-Pb and 39Ar-40Ar measurements on plutonic rocks recovered from the Ocean Drilling Program (ODP) Legs 173 and 210. Drilling revealed continental crust (Sites 1067 and 1069) and exhumed mantle (Sites 1070 and 1068) along the Iberia margin and exhumed mantle (Site 1277) on the conjugate Newfoundland margin. Our data record a complex igneous and thermal history related to the transition from rifting to seafloor spreading. The results show that the rift-to-drift transition is marked by a stuttering start of MORB-type magmatic activity. Subsequent to initial alkaline magmatism, localized mid-oceanic ridge basalts (MORB) magmatism was again replaced by basin-wide alkaline events, caused by a low degree of decompression melting due to tectonic delocalization of deformation. Such "off-axis" magmatism might be a common process in (ultra-) slow oceanic spreading systems, where "magmatic" and "tectonic" spreading varies in both space and time. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
