ABSTRACT Supermature siliciclastic sequences were deposited between 1.64 Ga and 1.59 Ga over a broad swath of southern Laurentia in the Archean, Penokean, Yavapai, and Mazatzal Provinces. These siliciclastic sequences are notable for their extreme mineralogical and chemical maturity, being devoid of detrital feldspar and ferromagnesian minerals, containing the clay mineral kaolinite (or its metamorphic equivalent, pyrophyllite), and having a chemical index of alteration >95. Such maturity is the result of a perfect confluence of tectonic and climatic conditions, including a stable continental crust with low topographic relief (the Archean, Penokean, and Yavapai Provinces ca. 1.70 Ga), a warm humid climate, an elevated level of atmospheric CO 2 , and relatively acidic pore fluids in the critical zone. The weathered detritus was transported and deposited by southward-flowing streams across the Archean, Penokean, and Yavapai Provinces, ultimately to be deposited on 1.66 Ga volcanic and volcaniclastic rocks in the Mazatzal continental arc along the southern margin of Laurentia.

ABSTRACT It is proposed that the Pinware orogen of eastern Canada, the Baraboo orogen of the midcontinent, and the Picuris orogen of the southwestern United States delineate a previously unrecognized, ~5000-km-long, ca. 1520–1340 Ma trans-Laurentian orogenic belt. All three orogenic provinces are characterized by Mesoproterozoic sedimentation, magmatism, metamorphism, and deformation—the hallmarks of a tectonically active plate margin. Tectonism was diachronous, with the earliest stages beginning ca. 1520 Ma in eastern Canada and ca. 1500 Ma in the southwest United States. Magmatic zircon age distributions are dominated by Mesoproterozoic, unimodal to multimodal age peaks between ca. 1500 and 1340 Ma. The onset of magmatism in the Pinware and Baraboo orogens was ca. 1520 Ma, and onset for the Picuris orogen was ca. 1485 Ma. Detrital zircon age distributions within each orogenic province yield maximum depositional ages between ca. 1570 and 1450 Ma. Minimum depositional ages generally fall between ca. 1500 and 1435 Ma, as constrained by crosscutting intrusions, metatuff layers, or the age of subsequent metamorphism. Metamorphic mineral growth ages from zircon, garnet, and monazite yield peak ages between ca. 1500 and 1350 Ma and tend to be older in the Pinware and Baraboo orogens than in the Picuris orogen. The 40 Ar/ 39 Ar cooling ages for hornblende, muscovite, and biotite yield significant peak ages between ca. 1500 and 1350 Ma in the Baraboo and Picuris orogens. We propose that the Pinware-Baraboo-Picuris orogen formed in a complex, diachronous, convergent margin setting along the southern edge of Laurentia from ca. 1520 to 1340 Ma.

ABSTRACT The Baraboo District includes an exceptional array of outcrops that have provided geological enlightenment to students and professionals, alike, for 150 years. In the late nineteenth century, several fundamental structural principles were developed here, such as criteria for determining stratigraphic facing and the significance of cleavage-bedding relations. More recent studies of deformational features in the folded Baraboo Quartzite, such as crenulation cleavage and quartz fabrics, have yielded insights into the kinematics of folding in the District and the significance of regional tectonics in the context of the Proterozoic assembly of North America. Additional petrologic, geochemical, and isotopic studies have established the age of the Baraboo Quartzite (≤1700 Ma), identified a Paleoproterozoic weathering profile, confirmed the supermature composition of the Baraboo Quartzite, established the presence of geon 14 hydrothermal alteration, and elucidated the Proterozoic tectonothermal evolution of the District, all of which bear importantly on Proterozoic tectonic, atmospheric, and climatic conditions in the southern Lake Superior region. By Late Cambrian time, the Baraboo Quartzite was a ring of islands, which was abutted by spectacular conglomerates deposited by tropical storms. These were surrounded by more distal sandstones and were eventually buried by Ordovician dolomite and sandstone. During the field trip, we will visit eleven localities, which have been selected to illustrate the key geological features of this North American classic.

Neogene doming in the north-central Klamath Mountains, California, tilted the Rattlesnake Creek terrane, chiefly an ophiolitic mélange, exposing an oblique cross section through disrupted and metamorphosed oceanic crust and mantle. The deepest section of the tilted terrane, in the Kangaroo Mountain area near Seiad Valley, contains tectonic slices of ultramafic, mafic, and sedimentary rocks that were penetratively deformed and metamorphosed under upper-amphibolite- to granulite-facies conditions. This section, called the Seiad complex, is the ophiolitic basement of an accreted Mesozoic island arc, and its polygenetic history reflects the magmatic and tectonic processes that occur during island-arc construction and evolution. The presence of metarodingite and metaserpentinite, and the concordance of structural elements and metamorphic grade among all units of the Seiad complex, indicate that initial tectonic disruption of the ophiolitic suite occurred in the upper crust and subsequent penetrative deformation and metamorphism occurred under high-temperature conditions in the deep crust. Crustal granulite-facies metamorphism is indicated by two-pyroxene metagabbroic bodies and two-pyroxene metasedimentary paragneiss. Geothermobarometric data from garnet amphibolite and granulite-facies metagabbro within the ophiolitic suite yielded pressure and temperature conditions of ~5–7 kb and ~650–750 °C. Geochemical data from samples of granulite, amphibolite, and leucotrondhjemite suggest a supra-subduction origin, although there is significant variation among the amphibolite samples, indicating multiple magma types. Crosscutting, radiometrically dated plutons and the regional geologic context suggest that high-grade metamorphism and deformation of these disrupted ophiolitic rocks occurred in the Middle Jurassic (ca. 172–167 Ma). This time interval broadly corresponds with contraction along several regional thrust faults in the Klamath Mountains province and juxtaposition of the Rattlesnake Creek terrane with terranes to the east. A U-Pb zircon age of 152.7 ± 1.8 Ma on a sample of a crosscutting leucotrondhjemitic dike swarm and published 40 Ar/ 39 Ar hornblende age spectra of ca. 150 ± 2 Ma from amphibolite indicate that magmatism and an accompanying thermal flux continued to affect this region into the Late Jurassic. Compared with the deep-crustal sections of the well-studied Kohistan and Tal-keetna arc complexes, the widespread mélange character of the Rattlesnake Creek terrane (including the Seiad complex) is unique. However, ophiolitic rocks, including mantle ultramafic rocks, are common components in the basal parts of these classic arc crustal sections. Hornblende gabbro/diorite and clinopyroxenite in the Seiad complex may be small-scale melt conduits that fed middle- and upper-crustal components of the arc, analogous to the relationship seen in Kohistan between deep-crustal ultramafic-mafic bodies and mid-crustal magma chambers.