The North American Cordilleran Anatectic Belt

The North American Cordilleran Anatectic Belt (CAB) is a ~3,000 km long region in the hinterland of the Cordillera that comprises numerous exposures of Late Cretaceous to Eocene intrusive rocks and anatectic rocks associated with crustal melting. As such, it is comparable in size and volume to major anatectic provinces including the Himalayan leucogranite belt. The CAB rocks are chiefly peraluminous, muscovite-bearing leucogranite produced primarily by anatexis of Proterozoic to Archean metasedimentary rocks. The CAB rocks lack extrusive equivalents and were typically emplaced as thick sheets, laccoliths, and dike/sill complexes. The extent, location, and age of the CAB suggests that it is integral to understanding the tectonic evolution of North America, however, the belt is rarely considered as a whole. This paper reviews localities associated with crustal melting in the CAB and compiles geochemical, geochronologic, and isotopic data to evaluate the melt conditions and processes that generated these rocks. The geochemistry and partial melting temperatures (ca. 675–775 °C) support water-absent muscovite dehydration melting and/or water-deficient melting as the primary melt reactions and are generally inconsistent with water-excess melting and high-temperature (biotite to amphibole) dehydration melting. The CAB rocks are oldest in the central U.S. Cordillera and become younger towards both the north and south. At any single location, partial melting appears to have been a protracted process (≥10 Myr) and evidence for re-melting and remobilization of magmas is common. End-member hypotheses for the origin of the CAB include decompression, crustal thickening, fluid-flux melting, and increased heat flux from the mantle. Different parts of the CAB support different hypotheses and no single model may be able to explain the entirety of the anatectic event. Regardless, the CAB is a distinct component of the Cordilleran orogenic system

Mesozoic to Cenozoic magmatic history of the Pamir

New geochronologic, geochemical, and isotopic data for Mesozoic to Cenozoic igneous rocks and detrital minerals from the Pamir Mountains help to distinguish major regional magmatic episodes and constrain the tectonic evolution of the Pamir orogenic system. After final accretion of the Central and South Pamir terranes during the Late Triassic to Early Jurassic, the P amir was largely amagmatic until the emplacement of the intermediate (SiO2 > 60 wt.%), talc-alkaline, and isotopically evolved (-13 to -5 zircon epsilon Hf-(t)) South Pamir batholith between 120-100 Ma, which is the most volumetrically significant magmatic complex in the Pamir and includes a high flux magmatic event at similar to 105 Ma. The South Pamir batholith is interpreted as the northern (inboard) equivalent of the Cretaceous Karakoram batholith and the along-strike equivalent of an Early Cretaceous magmatic belt in the northern Lhasa terrane in Tibet. The northern Lhasa terrane is characterized by a similar high-flux event at similar to 110 Ma. Migration of continental arc magmatism into the South Pamir terrane during the mid-Cretaceous is interpreted to reflect northward directed, low-angle to flat-slab subduction of the Neo-Tethyan oceanic lithosphere. Late Cretaceous magmatism (80-70 Ma) in the Pamir is scarce, but concentrated in the Central and northern South Pamir terranes where it is comparatively more mafic (SiO2 2500 km across the Pamir and northern Qiangtang terrane in Tibet. All of these complexes are located directly south of the Tanymas-Jinsha suture zone, an important lithospheric and rheological boundary that focused mantle lithosphere deformation after India-Asia collision. Miocene magmatism (20-10 Ma) in the Pamir includes: 1) isotopically evolved migmatite and leucogranite related to crustal anataxis and decompression melting within extensional gneiss domes, and; 2) localized intra-continental magmatism in the Dunkeldik/Taxkorgan complex. (C) 2017 Elsevier B.V. All rights reserved.NSF [EAR-1419748, EAR-1450899, EAR-1338583]; Romanian Executive Agency for Higher Education, Research, Development and Innovation [PN-III-P4-ID-PCE-2016-0127]; EarthScope Award for Geochronology Student Research24 month embargo; published online: 16 November 2017This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

(U‐Th)/He and 4He/3He Thermochronology of Secondary Oxides in Faults and Fractures: A Regional Perspective From Southeastern Arizona

Abstract Fe‐ and Mn‐oxides are common secondary minerals in faults, fractures, and veins and potentially record information about the timing of fluid movement through their host rocks. These phases are difficult to date by most radioisotopic techniques, but relatively high concentrations of U and Th make the (U‐Th)/He system a promising approach. We present new petrographic, geochronologic and thermochronologic analyses of secondary oxides and associated minerals from fault zones and fractures in southeastern Arizona. We use these phases in attempt to constrain the timing of fluid flow and their relationship to magmatic, tectonic, or other regional processes. In the shallowly exhumed Galiuro Mountains, Fe‐oxide (U‐Th)/He dates correspond to host‐rock crystallization and magmatic intrusions from ca. 1.6 to 1.1 Ga. Step‐heating 4He/3He experiments and polydomain diffusion modeling of 3He release spectra on these samples are consistent with a crystallite size control on He diffusivity, and little fractional loss of radiogenic He since formation in coarse‐grained hematite, but large losses from fine‐grained Mn‐oxide. In contrast to Proterozoic dates, Fe‐ and Mn‐oxides from the Catalina‐Rincon and Pinaleño metamorphic core complexes are exclusively Cenozoic, with dates clustering at ca. 24, 15, and 9 Ma, which represent distinct cooling or fluid‐flow episodes during punctuated periods of normal faulting. Finally, a subset of Fe‐oxides yield dates of ca. 5 Ma to 6 ka and display either pseudomorphic cubic forms consistent with oxidative retrogression of original pyrite or magnetite, or fine‐grained botryoidal morphologies that we interpret to represent approximate ages of recrystallization or pseudomorphic replacement at shallow depths