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Early Proterozoic structural geology, metamorphism, and effects of pluton emplacement, southern Big Bug Tectonic Block, Hieroglyphic Mountains, central Arizona

Jonathan Legg Burr, University of Massachusetts Amherst


The Hieroglyphic Mountains form a northwest-southeast transect across the southern part of the Big Bug tectonic block (Yavapai Province) including the block-bounding Shylock shear zone. Litho-stratigraphic correlations suggest a stratigraphic sequence composed of mafic metavolcanic rocks overlain by phyllites and the Pike's Peak banded iron formation unconformably overlain by schists and rhyolitic meta-arenites. Intrusive igneous rocks include foliated, northwest-striking biotite $\pm$ muscovite granite plutons in the central region (1.70 Ga Crazy Basin Quartz Monzonite equivalent?), and northeast-striking, foliated, biotite $\pm$ muscovite granite in the northwest (1.682 Ga Horse Mountain granite equivalent?). The first phase of deformation locally exhibits northwest-striking foliation axial planar to recumbent folds and west-directed shear fabrics. The second phase of deformation is represented by heterogeneously developed northeast-striking and steeply northwest-dipping crenulation cleavage and mylonitic foliation, folds that plunge gently northeast in low strain zones and steeply southwest in higher strain zones. Kinematics are variable from oblique northwest-side-up (dominant), to southeast-side-up (sinistral) movement. Second phase microstructures, including northwest-striking matrix fabric, and peak mineral assemblages are spatially related to the central granites. Geothermobarometry and P-T path calculations indicate near isobaric conditions (3.0 kbar) and a temperature range of ca. 400 to 600$\sp\circ$C. Uniform pressure estimates indicate little vertical movement across the Skylock shear zone. Temperature estimates and the distribution of biotite through sillimanite zone mineral assemblages indicate a thermal gradient spatially associated with the granite plutons. Textures and chemical zoning in garnet suggests two stages of growth consistent with a zone of aureole overlap between central and northwestern granites. Extensive development of andalusite-staurolite assemblages and northwest-striking, moderately-dipping fabrics in the central region are attributed to the thermal, rheologic and stain shadow effects in the roof zone of the pluton. Other domains of lower grade and upright fabric are interpreted to be pluton margin environments. The Shylock shear zone may obscure important early convergent structures and it may typify the strain partitioning in supracrustal rocks between major zones of plutonic rocks, particularly as a response to thermally enhanced shortening deformation during the Yavapai orogeny at ca. 1.70 Ga.

Subject Area


Recommended Citation

Burr, Jonathan Legg, "Early Proterozoic structural geology, metamorphism, and effects of pluton emplacement, southern Big Bug Tectonic Block, Hieroglyphic Mountains, central Arizona" (1992). Doctoral Dissertations Available from Proquest. AAI9305806.