Convection in a partially molten metasedimentary crust? Insights from the El Oro complex (Ecuador)

The El Oro complex, southwestern Ecuador, is a tilted section of the metasedimentary Ecuadorian forearc, which was partially molten during Triassic time due to gabbroic magma emplacement. Pressure and maximum temperature estimates show that the metamorphic gradient during anatexis was 45 °C/km in the upper crust and 10 °C/km in the 7–8 km garnet-bearing migmatitic lower crust, controlled by biotite-breakdown melting reactions. Our petrological and geochemical studies indicate that melts produced during biotite-breakdown (5–15 vol%) were trapped and pervasively distributed in the garnet-bearing migmatite. Based on these results we carried out one-dimensional thermal modeling to characterize the heat transfer processes that led to the establishment of such a low thermal gradient during partial melting. Our results show that neither diffusive nor upward melt transfer models account for the low metamorphic gradient in the garnet-bearing migmatite. We demonstrate that in the El Oro complex, convection of the garnet-bearing migmatitic layer is the most likely heat transfer process that explains all the petrological, geochemical, and metamorphic data. INTRODUCTION Both the largest Himalayan-Tibet and the Altiplano-Puna orogenic systems exhibit multiple evidence of ongoing partial melting in the middle to lower crust (e.g., Caldwell et al., 2009; Yuan et al., 2000). The genesis and rheology of such high-temperature middle to lower crustal sequences are therefore key to better understand the evolution of large orogenic systems. Because well-preserved deep sections of the crust are rarely exposed, and commonly exhibit multiple events of partial melting, most of the data from these zones are based on chemical investigation of outcrops of volcanic and shallow plutonic bodies (e.g., Mahéo et al., 2009), numerical modeling (e.g., Bittner and Schmeling, 1995), metamorphic studies of xenoliths (e.g., Ding et al., 2007), or Rayleigh wave dispersion studies (e.g., Caldwell et al., 2009). However, such sections are exposed in some key areas where they may be investigated. The El Oro complex, located in southwestern Ecuador (Fig. 1A), is a well-preserved crustal section that underwent a single event of mafic magma underplating and consequent crustal melting (Riel et al., 2013). To study the thermochemical evolution of the El Oro complex during partial melting, we used (1) structural, petrological, and geochemical tools to characterize the chemical exchanges in the crust; (2) available and new pressure and maximum temperature estimates (P-Tmax); and (3) thermal modeling using previous results to constrain the heat transfer mechanisms and their rheological implications. Our study suggests that in the middle crust, whole-rock convection with low melt content (5–15 vol%) might occur during biotite-breakdown melting reactions at 780–900 °C. This can potentially account for the formation of large volumes of partially molten middle crust in large orogenic systems. Field photographs and detailed methodology for P-Tmax estimates and geochemical and thermal modeling are provided in the GSA Data Repository1. GEOLOGICAL SETTING The El Oro complex is a tilted forearc section composed of unmetamorphosed pelites and sandstones to the south that progressively grade to a migmatitic series to the north, juxtaposed northward by a gabbroic pluton and a blueschist unit (Fig. 1B). Both metamorphic and magmatic events, which include gabbro and the granitoid emplacement, occurred in the Late Triassic between 230 and 225 Ma (Riel et al., 2013). The intrusion of mid-oceanic ridge basalt–type gabbro at the crustal root level triggered partial melting of the overlying metasedimentary sequence and emplacement of the granitoid belt in the unmolten metasedimentary upper crust (Fig. 2). Tectonic underplating of the blueschist unit at 226 ± 1.8 Ma (Gabriele, 2002) rapidly cooled both the gabbro pluton and the crust and marked the end of the anatectic event, allowing preservation of rare structures. The granitoid belt that intrudes the unmolten metasedimentary unit is 1–2 km thick and composed of amphibole + biotite granodiorite and muscovite + biotite monzogranite with numer1 GSA Data Repository item 2016008, photographs of the petrological study, geochemical methods, and detailed thermal and thermodynamic modeling methods, is available online at www.geosociety .org /pubs/ft2016.htm, or on request from editing@ geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. GEOLOGY, January 2016; v. 44; no. 1; p. 1–4 | Data Repository item 2016008 | doi:10.1130/G37208.1 | Published online XX Month 2015 © 2015 eological Society of A erica. For permission to copy, contact [email protected]. preatectic grdient La Victoria L. La Bocana M et as ed im en ts