SAME PACKAGE, DIFFERENT PATHS: COMPARATIVE PETROCHRONOLOGY OF THE GRAND CANYON’S PRECAMBRIAN VISHNU SCHIST FROM SUBDUCTION TO COLLISION USING IN-SITU MONAZITE EMPA DATES & PSEUDOSECTION MODELLING

Ancient orogens provide a window into the dynamic history of the earth’s continental crust, revealing intricacies between heat, pressure, time and tectonic processes that shape the planet’s surface and its geological resources. We investigate orogenic evolution during the Paleoproterozoic Yavapai Orogeny by integrating new pressure-temperature-time (P-T-t) paths from the Vishnu Schist within the Upper Granite Gorge, Grand Canyon, USA. We combine EMPA mineral chemistry, zonation data, and pseudosection modelling with in-situ monazite (mnz) EMPA total-Pb dates from two adjacent tectonometamorphic blocks with the same depositional sequence but variable reported peak P-T conditions.

The Clear Creek block (CCb) metaturbidites show no evidence of in-situ melting, preserving both protolith and early, subhorizontal D1 deformation fabrics, that have been variably overprinted by a high-strain, vertical fabric (D2). In contrast, D2 dominates the migmatitic and pegmatite-intruded Vishnu Schist of the adjacent Mineral Canyon block (MCb). Mnz across both blocks is associated with more pelitic domains and appear in the metamorphic fabric in conjunction with Fe-rich garnet, white mica, quartz and biotite. CCb mnz are much smaller (<25µm) than MCb mnz (>75µm).

The oldest CCb mnz (1.757–1.735 Ga) have high-Y concentrations (15wt%) and appear as either grains fully included within staurolite or as cores in zoned matrix mnz. Matrix mnz EMPA maps show Y gradually decreasing toward rims between 1.735–1.719 Ga, indicating onset of prograde garnet growth, corroborated by high-Y garnet cores.

Conversely, MCb monazite exhibit complex, opposite Y-zoning patterns over similar time frames. Low-Y cores (<5wt%) yield the oldest MCb mnz dates (c.1.750-1.735Ga), suggesting prograde growth with xenotime or garnet. Mnz Y jumps to >20wt% (1.735 – 1.712 Ga), before decreasing to ~8wt% (1.727-1.712 Ga). Mnz in MCb leucosomes linked to partially-resorbed garnets have young (c. 1.708 – 1.680 Ga), high-Y (>20wt%) rims that microstructurally appear to be syn-kinematic with D2 fabric development.

Continued petrologic modelling and in-situ petro- and deformation-chronometry will reveal the conditions and tempos under which the continental crust along the Paleoproterozoic margin of Laurentia was stabilized during the Yavapai Orogeny and the reasons for variable anatexis in the Vishnu Schist.