Accreted Eclogites with Oceanic Plateau Basalt Affinities in Ecuador

Abstract

High-pressure, regionally-metamorphosed rocks (the Raspas Formation) outcrop extensively (more than 10 km long and 3 km wide) within a terrane including extensively serpentinized harzburgite in E 1 0 r o Province, Ecuador. The high-pressure rocks consist of pelitic schists, eclogite, eclogite amphibolite and mafic rocks retrograded into amphibolites and greenschists. Exhumation of these high-pressure rocks would have occurred at about 132 Ma, based on K/Ar ages obtained from phengite (Feininger, 1980). Eclogites are composed chiefly of euhedral garnet, omphacite, black stubby amphibole and quartz. The garnet is peppered with abundant rutile inclusions, pyroxene and tiny quartz crystals. This metamorphic assemblage equilibrated under conditions of about P(total) = 1.3 GPa 1.5-2 GPa ( 45-60 km depth) and a T of 600 ~ On the basis of the major element compositions, the protoliths of the eclogites were basaltic (e.g. 97CE1:SiO2 = 49%; MgO = 7.58%; TiO2 = 1.48%; Fe203 =12.44%) and/or cumulate gabbroic (e.g. 97CE4A: SiO2 = 47%, MgO = 10.36%; TiO 2 = 0.46%; Fe203 = 8.12%). Trace element abundances have been determined both by bulk solution ICP-MS as well as laser ablation-ICP-MS analysis of individual ( 50 gm spots) mineral grains. The metabasalt has slightly enriched light rare earth element (LREE) abundances relative to heavy (H)REE [chondrite-normalised (cN) (La/Yb) CN = 1.3] with a slight negative Eu anomaly. In contrast, the metagabbro is characterised by depleted LREE abundances relative to HREE [(La/Yb)cN = 0.54] and a marked positive Eu anomaly. Relative to N-MORB, the metabasalt is Nb-, Ta-, Th-, U-, La-, and Ceenriched, and Srand Rbdepleted. In contrast, the gabbro is depleted in most of these the elements, additionally in Nb, Zr, and Hf, but has significant enrichments in Ba and Sr. Omphacite and amphibole separates have complementary REE abundance patterns, i.e. amphibole is depleted in LREE relative to the HREE [(La/Yb) cN = 0.41] while omphacite is enriched [(La/Yb)cN = 5.95]. Absolute REE concentrations of amphibole and omphacite are low (about 10 times chondritic abundances). In terms of isotopic characteristics, omphacite has an eNd(T=lSOMa) = +7.96, which is slightly higher than that of the amphibole (+6.82), but both ratios fall within the range of OIB. The metabasalt has a relatively high (87Sr/86Sr)i [0.70649; eSr(T-lS0Ma) = +30] which is likely related to oceanic hydrothermal alteration. Remarkably, the trace element abundances (absolute and relative) and isotopic characteristics of both eclogitised basalt and gabbro do not differ significantly from those of unmetamorphosed equivalents of an oceanic plateau origin, elsewhere in Ecuador (e.g. basalts and diabases of the Pifidn Formation, San Juan cumulate gabbro; refer to figure). We note further that the subcreted portion of the accreted