Magmatic source and metamorphic grade of metavolcanic rocks from the Granjeno Schist: was northeastern Mexico a part of Pangaea?

Abstract

The geochemistry of the metavolcanic rocks from the Granjeno Schist in northeastern Mexico indicates an origin in different tectonic environments: mid‐ocean ridge and ocean island. High ratios of Hf/Th and Th/Nb (4.4–14 and 0.08–0.15), low ratios of LaN/YbN and LaN/SmN (0.74–1.7 and 0.60–1.4) and depleted LREE patterns in metabasalt display mid‐ocean ridge characteristics. In contrast, the pattern of trace‐element ratios and REEs in metabasalt and metapillow lava 60 km to the west indicates a magma source with ocean‐island basalt characteristics. Both areas were metamorphosed during the Late Carboniferous (300 ± 4 Ma). Estimated metamorphic conditions deduced from white mica and chlorite compositions, distinguish greenschist facies (350 °C and 4 kbar) for the mid‐ocean ridge basalt, and prehnite–pumpellyite facies (250 °C and 2.5 kbar) for the ocean‐island‐type basalt. This metamorphism took place at an active continental margin during Pennsylvanian time. Our new tectonic model, which differs from earlier models, suggests that the origin of the Granjeno Schist is related to a subduction zone located at the western margin of Pangaea, active after Laurentia–Gondwana collision. Copyright © 2015 John Wiley & Sons, Ltd.

Petrology and geochemistry of meta-ultramafic rocks in the Paleozoic Granjeno Schist, northeastern Mexico: Remnants of Pangaea ocean floor

“…Instead, accretion and obduction in an evolving active margin is more plausible [19]. This is in accordance with the metamorphic age of the Granjeno Schist as well as with the presence of plagiogranite with syn-collisional and volcanic-arc characteristics, which intruded the Novillo Gneiss at c. 350 Ma [13,79].…”

“…Assuming that the youngest maximum depositional ages are similar to the youngest depositional part of the Granjeno Schist, N-MORB igneous activity and sedimentation in the Novillo and Peregrina Canyon area lasted at least from Silurian to Devonian times [19]. The extrusion of N-MORB from asthenospheric magma can be related to rifting and drifting of part of the Rheic Ocean during that time.…”

“…These rocks demonstrate that the closure of the Rheic Ocean, in the area of actual northeastern Mexico, did not result in continent-continent collision during the middle to late Paleozoic. It rather brought the northwestern margin of Gondwana into contact with the Paleo-Pacific Ocean [18,19,36,37].…”

“…The metamorphic age of the Granjeno Schist rocks of the Huizachal-Peregrina anticlinorium has been investigated in several studies. Most recently, [19] reported a 40 Ar/ 39 Ar plateau age on phengite of 300 ± 4 Ma in the metavolcanic rocks from the Peregrina Canyon. This age is equivalent with the reported age of 40 Ar/ 39 Ar in phengite of the metasedimentary rocks from the Novillo Canyon [13].…”

“…Most recently, Torres [19] suggest that N-MORB rocks indicates extrusion along a mid-ocean ridge related to the rifting and drifting of the Rheic Ocean during Silurian to Devonian time, whereas enriched basaltic rocks indicate mixing of MORB magma with an enriched deep mantle source. The magmatism was probably associated with plate movements related to the closure of the Rheic Ocean, supporting the migration of the southwestern margin of Gondwana into the PaleoPacific Ocean [20,21].…”