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Jun302004

Jade Tectonics in Central America

George E. Harlow Department of Earth and Planetary Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024-5192, USA

Sidney R. Hemming Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964-8000, USA

Hans G. Ave´ Lallemant & Virginia B. Sisson Department of Earth Science, MS-126, Rice University, Houston, Texas 77005-1892, USA

Sorena S. Sorensen Department of Mineral Sciences, NHB-119, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560-0119, USA

ABSTRACT
Left-lateral motion along the North American–Caribbean plate boundary has juxtaposed two high-pressure–low-temperature (HP-LT) belts from separate Cretaceous collisions. These two belts have quite different ages and different suites of high-pressure assemblages, yet they both contain jadeitite, a relatively rare rock type. This part of the plate boundary zone follows the Motagua River Valley in Guatemala, where it separates the Maya block (North American plate) from the Chortı´s block (Caribbean plate). On both sides of the bounding Motagua fault, tectonic slices of serpentinite-matrix me´lange host the HP-LT rocks. South of the fault, the me´lange slices contain eclogite, lawsonite eclogite, glaucophane eclogite, and blueschist blocks. North of the fault, the me´lange slices contain omphacite metabasite, albitite, and garnet amphibolite blocks, but lack intact eclogite. In addition to the dissimilar rock assemblages, 40Ar/39Ar geochronology of phengitic micas yields 77–65 Ma for northern and 125–113 Ma for southern blocks. These data suggest that the southern belt formed during Early Cretaceous (Aptian), northeastwarddipping subduction of the Farallon plate and collision of the Chortı´s block with western Mexico. The block was then displaced southeastward along this suture. In contrast, the northern belt records subduction related to the Maastrichtian collision of an extension of the Chortı´s block, perhaps the Nicaraguan Rise, with the Maya block.

Keywords: jadeitite, eclogite, serpentinite, plate boundary, suture zone, high pressure, low temperature, metamorphism, metasomatism, 40Ar/39Ar dating, Guatemala.


INTRODUCTION
The tectonics and history of the boundary between the North American and Caribbean plates in Central America are critical components for understanding the evolution of the Caribbean plate (e.g., Pindell, 1994; Dixon et al., 1998; Rogers et al., 2002). In Guatemala, the boundary is a zone of anastomosing leftlateral strike-slip faults that separate the Maya block of the North American plate from the Chortı´s block of the Caribbean plate, inclusive of the Nicaraguan Rise (Figs. 1 and 2). The three major strands of the boundary zone between the Caribbean plate and North American plate are, from north to south, (1) the Polochı´c-Chixoy fault; (2) the Motagua (San Agustı´n and Caban˜as)-Jubuco-Cuyamel fault; and (3) the Jocota´n-Chameleco´n fault (Fig. 1). Many serpentinite bodies are exposed along the Polochı´c and Motagua faults. Stratigraphic evidence suggests that some of these are parts of a dismembered ophiolite of Cretaceous age (Donnelly et al., 1990).

For 20 km on either side of the central Motagua River Valley, high-pressure–lowtemperature (HP-LT) rocks occur in fault slices between the Chortı´s and Maya blocks. About half the slices are serpentinite bodies, assigned by some to an ophiolite complex called the El Tambor Group (e.g., McBirney, 1963; Donnelly et al., 1990; Beccaluva et al., 1995). Others point out that all these bodies have faulted contacts, which may indicate slices of deep isolated peridotite (McBirney and Bass, 1969). Some of the serpentinite bodies contain jadeitite blocks. Jadeitite is a rare HPLT metamorphic rock that is globally associated with serpentinite (Harlow, 1994; Harlow and Sorensen, 2001). Jadeitite has been known from north of the Motagua (locally, Caban˜as) fault for more than 40 yr (Foshag and Leslie, 1955; McBirney et al., 1967; Harlow, 1994). In addition, eclogite cobbles have been described from the Rı´o El Tambor, a northflowing tributary to the Motagua River that drains serpentinites south of the Motagua fault (McBirney et al., 1967; McBirney and Bass, 1969; Smith and Gendron, 1997). Recent exploration for jade has yielded jadeitite in other serpentinite bodies, both north and south of the Motagua fault. Serpentinites south of the Motagua fault contain eclogite, glaucophane eclogite, blueschist, and lawsonite eclogite, as well as jadeite 1 pumpellyite, jadeite 1 quartz 6 rutile, jadeite 1 lawsonite, and lawsonite 1 omphacite 1 quartz rocks. The area distribution of jadeitites, omphacite metabasites, garnet amphibolites (some appear to preserve eclogite garnet), albitites, and related rocks north of the fault is much larger than previously recognized.

The age relationships of blocks are poorly constrained to unknown. However, phengitic muscovite is present in many of the HP-LT mineral assemblages. Therefore, 40Ar/39Ar geochronology was employed to determine the exhumation and crystallization ages of various rock types on both sides of the Motagua fault. Clusters of different mica ages for jadeitites and associated HP-LT rocks appear north and south of the Motagua fault zone. In this paper we discuss the origin and emplacement of the HP-LT rocks and revise the tectonic history of the North American– Caribbean plate boundary zone...


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