EFFECTS OF MAFIC INTRUSION ON TUFFACEOUS HOST ROCK, MEADOW CREEK BASIN, BLACK MOUNTAINS, AZ: FIELD, GEOCHEMICAL, AND PETROLOGIC EVIDENCE REGARDING FORMATION OF A GLASSY BORDER FACIES

Mafic to intermediate dikes and a mafic lopolith intrude the Miocene volcanic sequence of Meadow Creek basin, southern Black Mountains, AZ (Thorson 1971; Liggett & Childs 1982; & Schwat et al, Wallrich et al, Smith et al, Helfrich et al, [2016, GSA absts]). Contacts between these intrusions and the highly silicic, pumice-rich Sitgreaves Tuff (STG) and the Meadow Creek trachyte lava (MCL) display diverse effects of intrusions on host rocks and may provide insights on how magma input may mobilize country rock.

Contact effects are highly variable. At contacts, MCL is locally indurated but generally minimally affected by intrusion. STG at some contacts shows limited effects, but in general is strongly indurated. In many places there are patches of dense glass that we interpret to be fused or welded pumice fragments based on their sizes, shapes, and textures in thin section. Several locations are characterized by massive (> 10 cm thick), obsidian-like dense glass that encloses lithic fragments identical to those in the unmodified STG. Whole rock compositions (XRF) of STG (minimally altered, strongly indurated, and glassy) at contacts (69 to 76 wt % SiO₂) are identical to those of unmodified STG (Wallrich et al 2016). Dense, glassy material - both patches and massive - at contacts are compositionally similar to glass in unmodified SGT pumice (77 wt % SiO2, 3-5 ppm Sr, 4-12 ppm Ba, 250-300 ppm Rb (SEM-EDS, LA-ICPMS). Rhyolite-MELTS (Gualda & Ghiorso 2015) modeling of tuff margin contact glasses indicate a liquidus temperature of 1000 °C at time of dike emplacement for very low P (5.4 MPa based on depth of emplacement from the overlying stratigraphic thickness). Mafic dike temperatures are estimated at ~1200°C (Smith et al 2016).

Plausible explanations of the dense glasses present at contacts include: (1) extremely efficient heat transfer from intrusions leading to true melting of silicic pumice; (2) heating that raised temperatures locally to above the glass transition but was insuffient to produce melting; welding of pumice fragments and local rheomorphism was induced. Option (2) appears more likely, but in either case effectiveness of intrusion-related mobilization of silicic glass (without requirement for latent heat) is demonstrated (cf. Bindeman & Simakin 2014, Yellowstone; DuBray & Pallister 1999, Turkey Creek caldera, AZ).