RECONCILING "ANOROGENIC" PLUTONISM WITH REGIONAL CONVERGENCE AT ~1.4 GA: TRANSCURRENT TECTONICS IN SOUTHERN LAURENTIA

The origin of 1.4 Ga A-type plutons remains an outstanding problem in Precambrian geology. Geochemical studies have shown that 1.4 Ga plutons are most similar to rocks generated in rifts and along hotspots leading to the conclusion that they are anorogenic. In contrast, structural studies of 1.4 plutons have demonstrated horizontal shortening at a regional scale. Reconciling these two results is critical to understanding 1.4 Ga tectonics and the conditions that favor the generation of A-type magmas. In the Cimarron Mtns of northern New Mexico and the Wet Mtns of southern Colorado two transcurrent shear zones separated by ~200 km record sinistral top to the northeast normal shear. Both shear zones cut folds which record north to northwest directed shortening. The two shear zones deformed at amphibolite facies metamorphic conditions between 1430 Ma and 1390 Ma as recorded by 40/39Ar and U/Pb geochronometers. Neither shear zone is directly associated with a 1.4 Ga pluton suggesting they record regional deformation fields at ~1.4 Ga. Comparing the kinematics of these two shear zones suggests a regional transcurrent strain field with contemporaneous NW-SE shortening and NE-SW extension. This geometry is consistent with an E to NE trending dextral transform margin along southern Laurentia at ~1.4 Ga parallel to the belt of 1.4 Ga plutons. This interpretation is consistent with the strain field inferred from structural studies of 1.4 Ga plutons. Geochemical data from 1.4 Ga plutons is also consistent with a transcurrent strain field when compared to Phanerozoic A-type granites and other high K rocks. These rocks are often generated following convergent tectonics and are commonly associated with strike-slip faulting during transtension. We find this model appealing because horizontal stretching during transtension provides an environment favorable for the generation of A-type rocks during northwest directed shortening.