Thermal Constraints on the Emplacement Rate of a Large Intrusive Complex: The Manaslu Leucogranite, Nepal Himalaya

Abstract

The emplacement of the Manaslu leucogranite body (Nepal, Himalaya) has been modelled as the accretion of successive sills. The leucogranite is characterized by isotopic heterogeneities suggesting limited magma convection, and by a thin (<100 m) upper thermal aureole. These characteristics were used to constrain the maximum magma emplacement rate. Models were tested with sills injected regularly over the whole duration of emplacement and with two emplacement sequences separated by a repose period. Additionally, the hypothesis of a tectonic top contact, with unroofing limiting heat transfer during magma emplacement, was evaluated. In this latter case, the upper limit for the emplacement rate was estimated at 3Á4 mm/year (or 1Á5 Myr for 5 km of granite). Geological and thermobarometric data, however, argue against a major role of fault activity in magma cooling during the leucogranite emplacement. The best model in agreement with available geochronological data suggests an emplacement rate of 1 mm/year for a relatively shallow level of emplacement (granite top at 10 km), uninterrupted by a long repose period. The thermal aureole temperature and thickness, and the isotopic heterogeneities within the leucogranite, can be explained by the accretion of 20-60 m thick sills intruded every 20 000-60 000 years over a period of 5 Myr. Under such conditions, the thermal effects of granite intrusion on the underlying rocks appear limited and cannot be invoked as a cause for the formation of migmatites.

Higher Himalayan Shear Zone, Zanskar Indian Himalaya: microstructural studies and extrusion mechanism by a combination of simple shear and channel flow

“…The rate of ductile slip in the ZSZ was at least two times faster and ranged between 1.2 and 4.2 cm year -1 (Dèzes et al 1999). A similar order of rate of slip of 2 cm year -1 is given for the STDS (Annen et al 2006 and references therein). The combined activation of the boundaries of the HHSZ gave rise to fast extrusion of the HHSZ at an average rate of 5.0-8.8 mm year -1 (Dèzes 1999) along with extensive anatexis, migmatization, and leucogranite injection in the upper part of the HHSZ including the ZSZ (Walker et al 1999;Yin 2006 and references therein).…”

Factors Affecting the Thickness of Thermal Aureoles

“…According to a detailed analysis of the metamorphic assemblages and of the microstructures by Guillot et al (1995), the temperature remained high at the base of the pluton, whereas the cooling was more rapid at the top. The top aureole is <100 m. These characteristics suggest an incremental emplacement of the pluton by under-accretion (Annen et al, 2006). The thickness of the Manaslu leucogranite plution is estimated to between 5 and 12 km (Guillot et al, 1995).…”

“…The thickness of the Manaslu leucogranite plution is estimated to between 5 and 12 km (Guillot et al, 1995). The top aureole can be reproduced with numerical simulation if the leucogranite pluton results from the slow amalgamation of 10-100 m thick sills over more than 5 million years (Annen et al, 2006).…”

“…The thickness of thermal aureoles provides insights on magma emplacement timescales (Annen et al, 2006;Floess and Baumgartner, 2015). As emphasized by Wang (2012), caution must be exerted when inverting results and trying to recover magma emplacement conditions from the characteristics of thermal aureoles as different sets of parameters can lead to similar results.…”

Hypothesis for the origin of convergent margin granitoids and Earth’s continental crust by thermal migration zone refining

“…Long-held views concerning granitoid characterization and the relative importance of crustal versus mantle inputs have been questioned (Kemp et al, 2006). The discovery that individual plutons may take millions of years to form has led to new proposals that granitoids are emplaced in small increments Annen et al, 2006). Recently developed isotopic techniques have brought the surprising finding of consistently heavier iron isotopic ratios in igneous rocks of high SiO 2 content (Poitrasson and Freydier, 2005;Poitrasson, 2006;Schoenberg and von Blanckenburg, 2006;Heimann et al, 2008), providing stimulus to again address the question, ''How do granitoids form?"…”