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 Chortís block (Caribbean plate). On both sides of the bounding Motagua fault, tectonic slices of serpentinite-matrix mélange host the HP-LT rocks. South of the fault, the mélange slices contain eclogite, lawsonite eclogite, glaucophane eclogite, and blueschist blocks. North of the fault, the mélange slices contain omphacite metabasite, albitite, and garnet amphibolite blocks, but lack intact eclogite. In addition to the dissimilar rock assemblages, 40 Ar/ 39 Ar 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 Chortís 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 Chortís block, perhaps the Nicaraguan Rise, with the Maya block.
The interrelationships between metamorphism, deformation, magma intrusion, and 40Ar/39Ar geochronology were determined for a low‐pressure/high‐temperature metamorphic complex which formed from an accretionary prism in the Chugach Mountains, southern Alaska. Compressional deformation, which first produced south verging folds and associated thrusts, was followed by magma intrusion and development of north verging folds. Synmetamorphic southward directed thrusting of metamorphosed flysch over flysch produced increased load in the footwall, as documented by the distribution of mineral assemblages and by pressure‐temperature modeling of garnet growth. The initial heating to the greenschist facies may have been accomplished by a combination of advective heating from aqueous fluids and of conductive heating from subducted young oceanic crust. Regionally developed amphibolite facies metamorphism followed intrusion of felsic sills. The peak metamorphic conditions derived from geothermobarometry, mineral assemblages, and fluid inclusions ranged from 400° to 600°C at a depth of ∼10 km. The increased heat from associated synmetamorphic concordant felsic sills raised the ambient temperatures to produce a regional distribution of andalusite and cordierite with a core zone of sillimanite‐bearing migmatites. Subsequent cooling was initially rapid (≈55°C/Ma) to ∼350°C based on 40Ar/39Ar dates of 53 Ma for hornblende and 50 Ma for biotite and may have slowed to ∼11°C/Ma until 200°C based on an 40Ar/39Ar date of 35 Ma for plagioclase. Intrusions of felsic sills and at least one pluton were along the initially north dipping foliations associated with south verging folds. This suggests the source region for the melts may have been downdip in the subduction zone rather than from directly below within the accretionary prism.
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