THE EXTENSION OF THE PRECAMBRIAN MID-CONTINENT GRANITE-RHYOLITE TERRANE INTO TEXAS

The ca. 1.3-1.4 Ga Granite-Rhyolite terrane of the North American mid-continent extends southward into Texas where it is extensively reworked by Mesoproterozoic orogenesis that includes: 1) formation of a continental margin arc from ~1288-1232 Ma, 2) collision of an exotic arc and southern continent at ~1150-1116 Ma, and 3) intrusion of late syn- to post-tectonic granites between 1119-1070 Ma.

Field evidence for the Granite-Rhyolite terrane is found primarily in west Texas, where voluminous peraluminous meta-rhyolite flows and welded ash flow tuffs are interlayered with quartzites, meta-arkose, pelitic schist and meta-carbonate. A recent U/Pb age of 1332 +7/-3 Ma for a flow-banded meta-rhyolite verifies the general age range previously reported for these rocks. The supracrustal associations, lack of intrusive rocks, and previous geochemistry collectively suggest that these units are rift related. Metabasite sills are younger (1286 ± 3 Ma) and most likely unrelated. Only one dated unit within the Llano uplift of central Texas yields an age (1366+/3 Ma) that overlaps with the Granite-Rhyolite terrane proper. However, coupled with recent ages from basement cores, this unit helps tie the Llano Uplift to the mid-continent.

New mapping, U/Pb geochronology and geochemistry of the Valley Spring Gneiss of the Llano Uplift confirm a major period of magmatism consistent with an active margin from 1288 to 1232 Ma. Plutonic and volcanic rocks are primarily granititic, and recent geochemistry supports a continental arc-related origin. This interpretation is consistent with field relations that indicate the presence of a plutonic complex and volcanic rocks interlayered with volcanoclastic, clastic, and carbonate sedimentary rocks.

Subsequent reworking of this margin during the Grenville Orogeny from ~1150-1116 Ma, resulted in polyphase deformation along the southern margin and addition of a discrete exotic arc (dated at 1326-1275 Ma) in central Texas. Regional metamorphism varies from mid-amphibolite to greenschist facies in west Texas to upper amphibolite to granulite facies in the Llano uplift.