GREEN MOUNTAIN BLOCK, SIERRA MADRE (WYOMING) AND NORTHERN PARK RANGE (COLORADO): A PARTIAL CRUSTAL CROSS-SECTION THROUGH AN ACCRETED PALEOPROTEROZOIC ARC TERRANE?

The Green Mountain block (GMb), situated immediately south of the Archean Wyoming province, was accreted to the southern margin of Laurentia during the ~1.78–1.74-Ga Medicine Bow orogeny (Chamberlain, 1998). The GMb, the northern extent of the Colorado province, consists chiefly of the following components: (1) ~1.79-Ga Green Mountain Formation (GMF), (2) ~1.78-Ga Elkhorn Mountain Gabbro (EMG), (3) Big Creek Gneiss (BCG), and (4) various late syntectonic to post-tectonic Paleoproterozoic granitic rocks. The GMb is bounded on the north by the Cheyenne belt, and the Farwell Mountain-Lester Mountain suture zone (Tyson et al., 2002) is commonly considered its southern boundary. This composite block or terrane records a variety of petrologic and tectonic processes related to Paleoproterozoic crustal growth along the southern margin of Laurentia. It also may provide an oblique, partial crustal cross-section through an accreted Paleoproterozoic arc terrane. The deepest rocks in the proposed crustal section are exposed in the northern Park Range (Colorado) and include a heterogeneous suite of quartzo-feldspathic and amphibolitic gneisses with intercalated pelitic to calcareous metasedimentary rocks. Migmatization is widespread in this part of the GMb, and petrographic studies suggest dehydration-melting at high T–P conditions. To the north in the Sierra Madre (Wyoming), the BCG is the probable equivalent to the gneissic rocks in the northern Park Range, but this unit is apparently of lower metamorphic rank based on reduced migmatization, lower abundance of garnet in mafic rocks, and the lack of clinopyroxene as a peritectic phase. The BCG is interpreted as either a higher-grade metamorphic equivalent of the GMF or a fragment of an older basement of uncertain affinity. In either case, the GMF is structurally above the BCG. The EMG intrudes the GMF and correlative gabbroic rocks intrude the BCG. Subsequently, units 1–3 were intruded by late granitic rocks (unit 4), which may be related to partial melting of the BCG and other gneissic rocks during crustal thickening. The range of apparent crustal depths suggests that the GMb is an exceptional locality to study the crustal structure of a Paleoproterozoic arc as well as the processes associated with lateral tectonic accretion and magmatism along the southern margin of Laurentia.