BIMODAL CAMBRIAN HYPABYSSAL INTRUSIVE ROCKS AND ASSOCIATED BASALTIC PHREATOMAGMATIC VENT CONDUITS IN THE WEST TIMBERED HILLS, ARBUCKLE MOUNTAINS, SOUTHERN OKLAHOMA

Cambrian igneous rocks exposed in the West Timbered Hills (WTH) have not been mapped in detail previously. The main igneous unit in the WTH is the Carlton Rhyolite, which forms the uppermost part of the igneous fill within the Southern Oklahoma aulacogen (SOA). Our new work indicates that hypabyssal microgranite and diabase intrusions are also common in the WTH and are larger and more abundant than in most other volcanic outcrops in the SOA, suggesting the study area is closer to source vents for the volcanic rocks. Microgranite intrusions (68-72 wt. % SiO2) are ≤150 m across and show very similar trace element contents, indicating that the intrusions were sourced from a common magma chamber. Diabase intrusions (49-54 wt. % SiO2) occur as meter-scale dikes and larger discordant masses ≤270 m across. They form two distinct, petrogenetically unrelated groups on standard trace element discrimination diagrams. Group A intrusions plot in fields for alkaline or transitional basalts, along with diabases intruding rhyolite in the East Timbered Hills. Group B intrusions cluster tightly in tholeiitic fields.

A massive polymict igneous breccia is spatially associated with the microgranites and Group B diabases in at least two areas ~4 km apart, and mapping is in progress to determine if other breccia outcrops are present. The breccia cuts discordantly across Carlton Rhyolite and some hypabyssal intrusions, but other diabases intrude it. It contains rhyolite, microgranite, basalt and diabase clasts ≤1 m across in a finer fragmental matrix. The largest breccia body is ~1.4 km long; smaller masses are ~100-400 m across. Basaltic ash and fine lapilli in the matrix show features indicating generation by phreatomagmatic explosions, including abundant altered sideromelane and variably vesicular, angular to fluidal shapes. We interpret the breccia to fill conduits to maar volcanoes that formed when uprising mafic magma interacted explosively with groundwater in the volcanic sequence. Basalt and diabase clasts are geochemically identical to Group B diabase intrusions, some of which form fluidal, partly quench-fragmented tongues into the breccia, indicating they were intruded while the breccia was still wet and unlithified. These observations indicate that Group B intrusions were the driving force for the phreatomagmatism.