Geodynamic significance of the Raspas Metamorphic Complex (SW Ecuador): geochemical and isotopic constraints

Abstract

The Raspas Metamorphic Complex of southwestern Ecuador is regarded as the southernmost remnant of oceanic and continental terranes accreted in the latest Jurassic -Early Cretaceous. It consists of variably metamorphosed rock types. (1) Mafic and ultramafic rocks metamorphosed under high-pressure (HP) conditions (eclogite facies) show oceanic plateau affinities with flat REE chondrite-normalized patterns, eNd 150 Ma ranging from + 4.6 to 9.8 and initial Pb isotopic ratios intermediate between MORB and OIB. (2) Sedimentary rocks metamorphosed under eclogitic conditions exhibit LREE enriched patterns, strong negative Eu anomalies, Rb, Nb, U, Th, Pb enrichments, low eNd 150 Ma values (from À 6.4 to À 9.5), and high initial 87 Sr/ 86 Sr and 206,207,208 Pb/ 204 Pb isotopic ratios suggesting they were originally sediments derived from the erosion of an old continental crust. (3) Epidote-bearing amphibolites show N-MORB affinities with LREE depleted patterns, LILE, Zr, Hf and Th depletion, high eNd 150 Ma (> + 10) and low initial Pb isotopic ratios. The present-day well defined internal structure of the Raspas Metamorphic Complex seems to be inconsistent with the formerly proposed interpretation of a ''tectonic mélange''. The association of oceanic plateau rocks and continent-derived sediments both metamorphosed in HP conditions suggests that the thin edge of the oceanic plateau first entered the subduction zone and dragged sediments downward of the accretionary wedge along the Wadatti -Benioff zone. Subsequently, when its thickest part arrived into the subduction zone, the oceanic plateau jammed the subduction processes, due to its high buoyancy. In Ecuador and Colombia, the latest Jurassic -Early Cretaceous suture involves HP oceanic plateau rocks and N-MORB rocks metamorphosed under lower grades, suggesting a composite or polyphase nature for the latest Jurassic -Early Cretaceous accretionary event. D

A stable (Li, O) and radiogenic (Sr, Nd) isotope perspective on metasomatic processes in a subducting slab

“…Given the large natural variations in Sr-Nd isotope compositions and in Sr and Nd concentrations of subducting sediments and the fact that solid-fluid partition coefficient for sediment for Sr and Nd are close to 1 at appropriate temperatures of 650° and 700°C (Johnson and Plank, 1999), bulk mixing can be used as a simplifying model for comparative purposes. Admixing of a metasedimentary component as sampled in the Raspas complex (Bosch et al, 2002; this study) to a MORB-type eclogite produces mixing trajectories that come close to the composition of the zoisite vein and the zoisite eclogites. Terrigenous sediments, represented by sediments from the Lesser Antilles arc, possess the appropriate Sr-Nd isotope characteristics to produce zoisite eclogite compositions in Sr-Nd isotope space by mixing.…”

“…They have trace element characteristics suggesting an affinity to seamounts . The metasedimentary rocks are graphite-bearing garnet mica-schists interpreted as dominantly terrigenous sediments deposited at the Andean continental margin and incorporated into an accretionary prism (Bosch et al, 2002), but less silicic, metavolcanic schists also occur. The peridotites from the El Toro formation are serpentinized to variable degrees (≤ 10 to 95%).…”

“…Most importantly, the negative εNd values make a fluid component derived from subducted sediments most plausible. Although altered oceanic peridotites (Snow and Reisberg, 1995) have Sr-Nd isotope signatures similar to the zoisite eclogites, a sole contribution of an altered peridotite source to the Raspas zoisite eclogites is unlikely because of two reasons: First, harzburgite and serpentinite from the Raspas complex, which constitute a potential fluid source, do not show the required Sr-Nd isotope characteristics (Bosch et al, 2002). Second, extremely low Nd contents (< 1 ppm) and generally low Sr contents (< 10 ppm) in the altered peridotites would require a contributions of > 99% from this source to reach the zoisite eclogite composition if bulk mixing is assumed and > 90% if a 20fold enrichment of Sr and…”

Tectonic evolution of the Gulf of Mexico, Caribbean and northern South America in the mantle reference frame: an update

“…Strike-slip displacement of these terranes from southern Mexico may play a role in their exhumation prior to emplacement against the Yucatán Block later in the Cretaceous. The Raspas blueschist of southern Ecuador (Arculus et al 1999;Bosch et al 2002) may also originate at a west-facing trench.…”

International Lateinamerika-Kolloquium 2009

“…In contrast to this finding, Aspden et al (1992) reported a geotectonic event in the Late Cretaceous (65-85 Ma) as the latest datable event. If we would relate these Cretaceous to Tertiary ages to the above reported highpressure metamorphic event, we would conclude that more than just an oceanic plateau collided with northwestern South America as it was proposed, for instance, by Reynaud et al (1999), Bosch et al (2002) and Hughes and Pilatasig (2002) for Cretaceous to Palaeogene times. Alternatively, the high-pressure metamorphism could be older and, thus, would lead to the potential discovery of a so far unknown continental fragment which had collided with northwestern South America.…”