LATE PALEOZOIC HERCYNO-TYPE OROGENY, VENEZUELAN ANDES

Protoliths of the Late Paleozoic metasedimentary rocks which occur in the core of the Venezuelan Andes are thought to have been derived from an adjacent major block uplift. A regional zone of laminated staurolite schists, about 1 km thick, that encompasses andalusite, kyanite and sillimanite isograds, was overlain by about 1-2 km of greenschists, overlain in turn by about 11 km of epizonal and anchizonal flysch slates (and by diagenetic deposits now all removed). Pre-erosion reconstruction places the base of the staurolite schist zone at a depth of c.14-15 km and c.550°C.

Heretofore, coarser grained amphibolite facies quartzo-feldspathic rocks below the staurolite schists were considered to be part of the Precambrian Iglesias Complex. The contact between the two rock types is not clearly exposed but they appear to be structurally and thermally conformable. The massive 'layered' character of the underlying rocks is either remnant bedding or mimetic thereafter. Late Precambrian radiometric ages on the Valera granite, intrusive into the Iglesias Complex, indicates that some amphibolite grade metamorphic rocks are indeed of Precambrian age but other such rocks are likely of Late Paleozoic age and could be 2+ km thick.

Thus the Late Paleozoic sequence could have been c.16+ km thick. Despite local small-angle obliquity of the staurolite zone to formation contacts, overall metamorphic zonation is close to stratigraphy-parallel. This points to an auto-metamorphic process in which the thermal gradient in both the slate and the staurolite facies was about 30°-35°C/km. In the near-total absence of igneous rocks (intrusive granites are post-regional metamorphism) the heat source for the metamorphism cannot be stipulated. The enormous thicknesses of illite-rich flysch suggest that K-rich crust may have been doubled but such a radiogenic heat source would not have sufficed by almost half. The current ranges have been thrust over c.11 km of flysch-rich sediment in the Maracaibo basin; perhaps such tectonic loading led to the formation of this Late Paleozoic Hercyno-type orogen but no remnant structures have been identified. Investigation of this problem continues.