GRENVILLE OROGENESIS ALONG THE SOUTHERN LAURENTIAN MARGIN: CONTRASTS BETWEEN THE EASTERN AND WESTERN LLANO UPLIFT, CENTRAL TEXAS

Mesoproterozoic metamorphic rocks exposed in the Llano Uplift, central Texas, record a Grenville-age orogenic event at ~1150-1116 Ma Ga. Although the entire uplift shows a similar and coeval tectonic history, preliminary work in the western uplift indicates distinct differences from the well-studied eastern uplift, which provide insight into the evolution of this margin during Grenville orogenesis.

In the eastern uplift, the orogenic history is dominated by the collision of the exotic Coal Creek island arc terrane, which collided with the southern margin of Laurentia. Distinct lithotectonic domains are bounded by thick zones of mylonite and intense transposition of supracrusal rocks. Tectonic transport is to the northeast. Associated upper amphibolite to granulite facies metamorphism increases with structural depth, which is to the northeast.

In marked contrast, in the western uplift no remnant of the Coal Creek arc terrane has been found. Major lithotectonic domains are not juxtaposed by shear zones, and the intensity of deformation does not increase near the boundaries. Tectonic transport is generally to the southwest, opposite that of the eastern uplift. Structures in the western uplift are more easterly or northeasterly striking than the dominantly southeast-striking structures in the eastern uplift. Associated upper amphibolite to granulite (transitional to eclogite) facies metamorphism increases with structural depth, which is to the southwest, opposite that of the eastern uplift.

All of the uplift records a similar, essentially coeval, polyphase deformational, metamorphic, and magmatic history. However, only the youngest fold generation can be directly correlated across the entire uplift, suggesting that the local kinematic histories diverged until that time. The difference in kinematic histories is best explained by the presence of the Coal Creek arc terrane in the southeastern uplift. We propose that the kinematic evolution of the eastern uplift was directly related to the northeastward collision of the arc terrane with the generally east-trending Laurentian margin, whereas deformation in the western uplift was controlled by collision of a southern continental block. Further work in the western uplift is needed to better constrain the differences in the evolution of the eastern and western uplift.