Introduction Ecuador is part of the northern Andes, which comprise accreted oceanic terranes on their western part. In Ecuador, these terranes are underplated, and form the crustal root of the Western Cordillera (Guillier et al. 2001). On the other hand, Recent arc magmatism is marked by abundant volcanic rocks of adakitic affinity (Bourdon et al. 2002, 2003). This led to the hypothesis that the adakitic products derive from the partial melting of the deeply buried accreted oceanic material (Arculus et al. 1999, Beaudon et al. 2005). The aim of this paper is to present new geochemical results on Miocene intrusions sampled in the Western Cordillera (Fig. 1), in order to check wether adakites occur, and if so, to constrain their origin.
This study reassesses the development of compositional layering during the growth of granitic plutons, with emphasis on fractional crystallization and its interaction with both injection and inflation-related deformation. The Dolbel batholith (SW Niger) consists of 14, kilometre-sized plutons emplaced by pulsed magma inputs. Each pluton has a coarse-grained core and a peripheral layered series. Rocks consist of albite (An≤11), K-feldspar (Or96–99, Ab1–4), quartz, edenite (XMg = 0·37–0·55), augite (XMg = 0·65–0·72) and accessories (apatite, titanite and Fe–Ti-oxides). Whole-rock compositions are metaluminous, sodic (K2O/Na2O = 0·49–0·62) and iron-rich [FeOtot/(FeOtot + MgO) = 0·65–0·82]. The layering is present as size-graded and modally graded, sub-vertical, rhythmic units. Each unit is composed of three layers, which are, towards the interior: edenite ± plagioclase (Ca/p), edenite + plagioclase + augite + quartz (Cq), and edenite + plagioclase + augite + quartz + K-feldspar (Ck). All phases except quartz show zoned microstructures consisting of external intercumulus overgrowths, a central section showing oscillatory zoning and, in the case of amphibole and titanite, complexly zoned cores. Ba and Sr contents of feldspars decrease towards the rims. Plagioclase crystal size distributions are similar in all units, suggesting that each unit experienced a similar thermal history. Edenite, characteristic of the basal Ca/p layer, is the earliest phase to crystallize. Microtextures and phase diagrams suggest that edenite cores may have been brought up with magma batches at the site of emplacement and mechanically segregated along the crystallized wall, whereas outer zones of the same crystals formed in situ. The subsequent Cq layers correspond to cotectic compositions in the Qz–Ab–Or phase diagram at PH2O = 5 kbar. Each rhythmic unit may therefore correspond to a magma batch and their repetition to crystallization of recurrent magma recharges. Microtextures and chemical variations in major phases allow four main crystallization stages to be distinguished: (1) open-system crystallization in a stirred magma during magma emplacement, involving dissolution and overgrowth (core of edenite and titanite crystals); (2) in situ fractional crystallization in boundary layers (Ca/p and Cq layers); (3) equilibrium ‘en masse’ eutectic crystallization (Ck layers); (4) compaction and crystallization of the interstitial liquid in a highly crystallized mush (e.g. feldspar intercumulus overgrowths). It is concluded that the formation of the layered series in the Dolbel plutons corresponds principally to in situ differentiation of successive magma batches. The variable thickness of the Ck layers and the microtextures show that crystallization of a rhythmic unit stops and it is compacted when a new magma batch is injected into the chamber. Therefore, assembly of pulsed magma injections and fractional crystallization are independent, but complementary, processes during pluton construction.
Les similitudes observees entre les enclaves microgrenues sombres des deux granites etudies revelent un processus de formation unique. qui peut etre etendu aux enclaves d'autres massifs intrusifs calco-alcalins. L'interpretation suivante est proposee: - les enclaves microgrenues sont des roches ignees temoignant de la coexistence de magmas de composition contrastee qui ont interagi de maniere complexe a differents stades de leur evolution respective selon le processus general suivant: - mise en contact d'un magma basique s.l. chaud et peu cristallise avec un magma granitique partiellement cristallise en base de chambre magmatique, - melanges mecaniques repetes et en proportions variables conduisant a la destabilisation des phenocristaux du magma granitique (plagioclases a patchy-zoning, feldspaths alcalins a couronne rapakivi, ocelles de quartz a lisere de ferromagnesiens) dans les magmas hybrides resu ltant, echanges chimiques selcctifs favorises par la presence de fluides, -injection et dispersion en enclaves des magmas hybrides dans les parties superieures plus froides et visqueuses de la chambre magmatique, - cristallisation rapide des enclaves jusqu'a disparition du contexte de surfusion, echanges chimiques intenses avec le granite sous controle mineralogique et en presence de fluides (enrichissement des enclaves en alcalins et ions a forte densite de charge, developpement d'une bordure centimetrique sombre), - fin de cristallisation plus lente en parallele avec celle du granite. Le granite miocene du Monte Capanne resulte de melanges entre un magma anatectique dominant d'origine crustale metapelitique et un magma basaltique mantellique represente par les enclaves dans un contexte distensif postsubduction. Le granite tardi-varisque du Mont-Blanc contient deux familles d'enclaves (magnesiennes et ferriferes) distinctes, la seconde ayant des caracteres chimiques anormaux excluant l'identification definitive de sa source. Des filons tardifs de composition identique a celle des enclaves magnesiennes confirment l'origine magmatique de ces dernieres. Le granite du Mont-Blanc provient de la fusion d'une croute granulitique profonde en contexte distensif epirogenique favorisee par l'intrusion de magmas basiques d'origine mantellique dont l'existence est attestee par les enclaves et les filons. Le magma granitique comprend une eventuelle composante mantellique dont la proportion est inconnue. Les enclaves microgrenues des granitoides calco-alcalins en general sont considerees comme le temoin de la coexistence de magmas basique et acide non consanguins. mais cogenetiques dans le sens ou l'existence du premier a pu induire celle du second. Leur hybridation peut engendrer d'importants volumes de magmas de composition intermediaire.
Abstract Emerald from the Binntal occurrence in the Canton of Valais in Switzerland has been studied to determine its chemical zonation, stable isotopic signatures, depositional-fluid characteristics, pressure-temperature emplacement conditions, and formational model. The emerald is vanadium-rich, with optical and blue cathodoluminescence zoning related to chemical variations, primarily in V 2 O 3 concentrations. The hydrogen isotope signature of the emerald channel fluids is unique and in agreement with previously identified high-altitude (deuterium-depleted) Alpine-age meteoric fluids. Field studies, fluid inclusion analyses, and oxygen isotope thermometry are consistent with a metamorphic formational model for the Binntal emerald at temperatures and hydrostatic pressures ranging from 200 to 400 °C and 100 to 250 Mpa, respectively. This corresponds to formational depths on the order of 4 to 9 km and fluids consistent with a 10–20 Ma CO 2 -dominant fluid with approximate mole percentages of 84.0, 11.9, 1.5, 1.3, 0.3, and 0.5 for CO 2 , H 2 O, CH 4 , N 2 , H 2 S, and NaCl, respectively.
