Early Mesoproterozoic evolution of midcontinental Laurentia: Defining the geon 14 Baraboo orogeny

New geochronologic data from midcontinental Laurentia demonstrate that emplacement of the 1476-1470 Ma Wolf River granitic batholith was not an isolated igneous event,but was accompanied by regional metamorphism,deformation,and sedimentation.Evidence for such metamorphism and deformation is best seen in siliciclastic sedimentary rocks of the Baraboo Interval,which were deposited closely following the1.65-1.63 Ga Mazatzal orogeny.In Baraboo Interval strata,muscovite parallel to slatey cleavage,in hydrothermal veins,in quartzite breccia,and in metamorphosed paleosol yielded ~(40)Ar/~(39)Ar plateau age s of 1493-1465 Ma.In addition,U-Th-total Pb dating of neoblastic overgrowths on detrital monazite gave an age of 1488±20 Ma,and recrystallized hematite in folded metapelite gave a mean U/Th-He age of 1411± 39 Ma.Post-Baraboo,arkosic polymictic conglomerate,which contains detrital zircon with a minimum peak age of 1493 Ma,was intruded by a 1470 Ma granite porphyry at the northeastern margin of the Wolf River batholith.This episode of magmatism,regional deformation and metamorphism,and sedimentation,which is designated herein as the Baraboo orogeny,provides a midcontinental link between the Picuris orogeny to the southwe st and the Pinware orogeny to the northeast,completing the extent of early Mesoproterozoic(Calymmian) orogenesis for 5000 km along the southern margin of Laurentia.This transcontinental orogen is unique among Precambrian orogenies for its great width(～1600 km),the predominance of ferroan granites derived from partial melting of lower continental crust,and the prevalence of re gional high T-P metamorphism related to advective heating by granitic magmas emplaced in the middle to upper crust.     

Crystal clots in calc-alkaline andesites as breakdown products of high-Al amphiboles

Andesites of the calc-alkaline volcanic series associated with the circum-Pacific orogenic zone commonly contain crystal clots consisting essentially of plagioclase, clinopyroxene, orthopyroxene, and magnetite. It is proposed that these crystal clots represent the breakdown products of an amphibole as it enters the low-pressure environment of the upper crust. The bulk chemical composition of the clots compares favorably to that of the high-Al amphibole, pargasitic hornblende. The crystal clots support the hypothesis of the formation of andesitic magma by fractionation of early formed amphibole from a basaltic magma at total pressures less than 18 kbars and temperatures less than 1000° C. The origin of these clots has previously been attributed to random accumulation of phenocrysts. Some features of clot-bearing andesites from Crater Lake, Oregon, U.S.A., cannot be explained by this mechanism. First, in some andesites, certain minerals occur as phenocrysts but are not constituents of the clots, and conversely, certain minerals occurring as accessories in the clots are rarely found as phenocrysts. Second, the minerals comprising the clots occur in a fixed ratio that is significantly different than the ratio of the same minerals as phenocrysts. Crystal clots may form up to 10% by volume of the andesite, imparting a glomeroporphyritic texture to the rock. Crystal clots can be distinguished from xenoliths of similar mineralogy by the presence in the latter of abundant glass, both as interstitial material and as inclusions in the plagioclase grains, giving the plagioclase a “spongy” appearance.     

Formation processes affecting submerged archaeological sites:An overview

Although the formation processes operating on submerged archaeological sites are just as varied as those affecting terrestrial ones, nautical archaeologists have not yet devoted much attention to them. Most studies to date are concerned with formation processes at particular sites. This article provides an overview of the major depositional and postdepositional formation processes affecting underwater sites. The most obvious depositional process is shipwreck, which takes several different forms. Submerged sites may also be formed by the drowning of coastal areas due to tectonic or eustatic sea level changes. In these cases, rapid submergence preserves sites better than slow inundation, which allows time for waves and currents to tear the site apart. For both shipwrecks and coastal sites, once submergence occurs, the single most important factor for preservation is rapid burial by sediment. A cover of sediment protects both the artifacts themselves and their spatial patterning from destruction by water and marine organisms. Once deposited, underwater sites are subject to modification by both cultural and natural processes. The best understood postdepositional processes include salvaging, treasure hunting, and destruction by marine borers. Others, such as dredging, construction, and bioturbation, have hardly been investigated at this time. Archaeologists need to devote more attention to the effects of marine animals that live in close association with the seabed, as well as marine plants, whose roots may disturb sites located in shallow water. From this study it is clear that maritime archaeologists must consider formation processes when planning projects, rather than thinking of underwater sites as simply “time capsules.” © 1999 John Wiley & Sons, Inc.