Checking billy barr’s snow log…
The Needle Mountains are an uplifted basement block that resides along the Mazatzal-Yavapai crustal boundary. Amphibolite facies gneisses are the oldest rocks exposed in the range (1786-1706 Ma in age) and record two episodes of penetrative deformation during arc magmatism. The first deformation event produced a north-northwest striking subvertical foliation with shallow stretching lineation and steeply plunging fold hinges. The geometry of the structures indicates they most likely formed during transpression, although the kinematics of deformation are not constrained. The second deformation event in the gneiss produced east-northeast striking foliation with sub-horizontal lineation. Kinematic indicators are consistently left lateral, consistent with left lateral transpression. Peak metamorphic conditions were in the upper amphibolite facies at pressures of ∼8 kbar and temperatures near 700°C. Intrusion of late kinematic calc-alkaline granites occurred after the formation of the second fabric. Uplift and exhumation of these rocks occurred by ∼1700 Ma when quartzite and schist of the Uncompahgre group were deposited uncomformably on the gneiss. The combined package of gneiss and quartzite was then folded into a sequence of kilometer scale folds with subvertical axial surfaces and shallowly plunging hinges. Quartz c-axis patterns indicate deformation in the low amphibolite facies during predominately simple shear deformation parallel to bedding planes in the quartzite. Strain partitioning between quartzite and schist is indicated by variation of lineation plunge between the quartzite and schist. Kinematic and geometric analysis indicates the schist record more flattening and horizontal stretching then the adjacent quartzite. Metamorphic grade was upper greenschist to low amphibolite at pressures below 4 kbar. Monazite age dating shows these structures formed between 1700 Ma and 1600 Ma. Both the Uncompahgre group and basement gneiss were then intruded by 1435 Ma plutons, including the Eolus pluton, Electra Lake Gabbro, and Pine River Batholith. Intrusion of the pluton’s resulted in refolding of basement gneiss and cover producing a horse-shoe shaped map pattern, most prominent around the northern margin of the Eolus pluton. Intensity of folding increases towards the Eolus. Vein arrays, geometry of folds, localized shear zones and quartz c-axis fabrics indicate the granite intruded during approximately north-south shortening and east-west extension. A well-developed contact aureole is developed around the granite. Strain gradients indicate that deformation is more intense in the inner aureole than at distance from the pluton demonstrating that thermal softening caused localization of deformation around pluton. These events, which occurred over ∼400 Ma record the formation of the continental crust. They show that initially formed arc crust was exhumed and buried by a thick sedimentary section. Subsequent orogeny reburied these rocks and caused them to reside in the middle crust for 200 Ma until 1400 Ma granites intruded. This final event was the last major modification of the crust and was followed by cratonization of this part of North America.
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