The Journal of Geology

doi: 10.1086/629290pmid: N/A

Weathering reactions mainly involve the transformation of feldspars, phyllosilicates, amphiboles, pyroxenes, and volcanic glass to the secondary mineral groups, kandites, illites, smectites, vermiculites, and/or chlorites. Although mineralogical changes are complex, bulk compositional changes to weathering profiles, resulting from chemical weathering, are simple and predictable from kinetic, thermodynamic, and mass balance considerations. Predicted bulk compositional changes are corroborated by studies of Recent weathering profiles developed on a variety of plutonic and volcanic rocks under different climatic regimes. Unlike the mineralogical compositions of profiles, the bulk compositional trends are not noticeably modified by climate; consequently, the simple, predictable bulk compositional trends observed in recent profiles provide a "norm" to which ancient weathering profiles can be compared. Early diagenetic reactions may occur prior to burial of the profile by reaction of groundwaters with secondary weathering products. These often result in abnormally high accumulations of Si, $$CO_{2}$$, Ca, and Mg to form clay minerals (smectites) and carbonates. The accumulations may be used as indicators of (paleo-)water tables. Late diagenetic reactions occur during and following burial through reaction of basin waters, trapped seawater, or brines with minerals of profiles. Metasomatism is common and includes production of illites, smectites, and chlorites at the expense of kaolinite and reconstitution of partially degraded feldspars to form potash feldspar and albite. Reaction with seawater (high Na/K and Mg/K) results in Na- and Mg-metasomatism, yielding albite and chlorite at the expense of partially degraded feldspars and clay minerals. In contrast, K-metasomatism of buried weathering profiles is favored around the periphery of subsiding continental sedimentary basins where dilute continental ground waters display low Na/K values. NaCl-rich brines and high temperatures in the deep central parts of basins favor the formation of albite at the expense of K-feldspar.

doi: 10.1086/629291pmid: N/A

A geochronologic study of blueschists from west-central Baja California has provided constraints on the timing of high-pressure, low-temperature metamorphism and the subsequent uplift of this subduction complex. Mineral separates from coherent blueschists and blueschist blocks enclosed in serpentinite melange were analyzed by $$^{40}Ar/^{39}Ar$$, fission track, and Rb/Sr methods. $$^{40}Ar/^{39}Ar$$ age spectra for white mica separated from four blueschist blocks and an overprinted eclogite block in serpentinite-matrix melange show slow cooling gradients between 115-95 Ma. Rb/Sr apparent ages for one block are concordant with these results. $$^{40}Ar/^{39}Ar$$ ages of blue amphibole are primarily the result of the degassing of white mica intergrowths as confirmed by microprobe analyses. Apatite fission track ages indicate a blueschist block and an eclogite block cooled to a temperature below ~100°C at 22 Ma and 32 Ma, respectively. Subduction-related metamorphism of the coherent blueschists occurred in late Early Cretaceous time, based on a $$^{40}Ar/^{39}Ar$$ age of 109 Ma for metamorphic white mica from a metasandstone. $$^{40}Ar/^{39}Ar$$ analysis of partially albitized K-feldspar from a plutonic clast in a metaconglomerate indicates this sample cooled to a temperature of ~145°C at approximately 20 Ma. Geochronologic and petrologic data for coherent blueschists indicate an average uplift rate of 0.1 mm/yr for one portion of the subduction complex. The relatively slow uplift rate and lack of any higher temperature overprinting assemblages in the coherent blueschists suggest that synsubduction uplift was gradual and proceeded through a dynamic accretionary wedge characterized by low geothermal gradients. An increase in uplift rate (to 1.0 mm/yr) during post-Miocene time coincides with a change from a convergent to a transform plate boundary.

doi: 10.1086/629292pmid: N/A

Two groups of migmatites represent the thermal culmination of the classic area of Barrovian metamorphism in the Eastern Highlands of Scotland. Group A are spatially and temporally associated with kyanite-grade metamorphism, while Group B are spatially and temporally associated with sillimanite-grade metamorphism and structurally overprint Group A. Structural, textural, modal, and mineral chemical arguments indicate that Group A migmatites are the products of subsolidus differentiation, whereas Group B migmatites apparently were formed by anatexis. Kyanite metamorphism and Group A migmatites probably result from conductive heating related to regional-scale nappe emplacement locally modified by advection during synmetamorphic folding. Sillimanite metamorphism and Group B migmatites, on the other hand, result from local and transient fluid advection. The relative importance of conductive vs. advective heat transfer has apparently varied in both space and time over a relatively small ($$< 50 km^{2}$$) area.

doi: 10.1086/629293pmid: N/A

The St. Lucie Metamorphic Complex comprises part of the non-Laurentian, Paleozoic Suwannee terrane beneath the Gulf and Atlantic Coastal Plains of the southeastern United States. Hornblende concentrates prepared from cuttings from two deep test wells penetrating the complex display internally concordant $$^{40}Ar/^{39}Ar$$ incremental-release spectra defining plateau ages of $$510.8 \pm 1.1 Ma$$ and $$513.1 \pm 1.8 Ma$$, which are interpreted to date post-metamorphic cooling through temperatures required for intracrystalline argon retention. The Kasila Group constitutes the western segment of the Rokelide orogen in Sierra Leone. Four hornblende concentrates prepared from amphibolite within the Kasila Group yield $$^{40}Ar/^{39}Ar$$Ar plateau ages of $$505.0 \pm 5.2$$, $$508.2 \pm 2.1$$, $$510.5 \pm 2.6$$, and $$546.1 \pm 6.8 Ma$$. They are interpreted to date post-metamorphic cooling through appropriate argon closure temperatures following a ca. 550-560 Ma, Pan-African II phase of tectonothermal activity. A biotite concentrate from paragneiss within the Kasila Group displays an internally concordant $$^{40}Ar/^{39}Ar$$ release spectrum defining a plateau age of $$524.7 \pm 1.3 Ma$$, which likely reflects slight contamination with extraneous (excess) argon. Muscovite from the Marampa Group yields a $$^{40}Ar/^{39}Ar$$ plateau age of $$561.1 \pm 2.3 Ma$$, which is interpreted to date cooling through appropriate argon closure temperatures. Although these temperatures are generally considered to be lower than those for hornblende, the muscovite records an older cooling age, suggesting that the Marampa Group experienced slightly earlier post-metamorphic uplift and cooling relative to the Kasila Group. Lithologic comparisons combined with similarities in post-metamorphic thermal evolution suggest that the St. Lucie Metamorphic Complex originated within the Rokelide orogen. This and other lithotectonic elements of the Suwannee terrane appear to represent a fragment of Gondwana which accreted to Laurentia during late Paleozoic amalgamation of Pangea.

doi: 10.1086/629294pmid: N/A

The intimate coexistence of two microstructurally defined phyllosilicate populations that have apparently achieved two different degrees of diagenetic/metamorphic maturity underscores the importance of chemical processes in slaty cleavage development. Petrographic observations and analytical data on phyllites from Ocoee Gorge, Tennessee, indicate that most of the cleavage-parallel phyllosilicates concentrated in thin lamellae grew during low-grade metamorphism. On the other hand, the bedding-plane parallel phyllosilicates found between lamellae are detrital or diagenetically altered grains that were in place before slaty cleavage developed. Electron microprobe and XRD analyses have been used to characterize the mica, interstratified mica/chlorite, and chlorite found in the two microstructurally defined populations. Cleavage-parallel phyllosilicates are homogeneous and have crystal chemical characteristics indicative of low-grade metamorphism. Bedding-parallel phyllosilicates, on the other hand, are more heterogeneous and appear to be detrital or diagenetic. Specific differences in the two groups include greater K content of cleavage-parallel mica and the greater chemical variability of bedding-parallel mica. Cleavage-parallel interstratified mica/chlorite is limited to a maximum of about 50% chlorite, whereas there is no limit on chlorite in bedding-parallel interstratified grains. In addition, the proportion of mica in the total phyllosilicates is higher parallel to cleavage than parallel to bedding. The differences between the cleavage-parallel and bedding-parallel phyllosilicates suggest that new grain growth and recrystallization of pre-existing grains were more important than passive concentration and rotation in the development of this fabric.

doi: 10.1086/629295pmid: N/A

Small (<150 m high) volcanic cones and larger oceanic central volcanoes (seamounts) up to 600 m high are abundant near the Mid-Atlantic Ridge ~26°S. They are present along the axis, within the rift valley floor, and also on the ridge flanks. Seabeam mapping of $$~16,500 km^{2}$$ within about 130 km of the axis (~7 Ma) shows up to 118 seamounts and cones for a normalized abundance of $$~7000/106 km^{2}$$. Their abundance is highest near the axis and decreases outward on the flanks of the ridge. Small cones and seamounts are much more abundant than large ones. They range in shape from simple cones with flat summits to more complex flat-topped volcanoes with summit craters, arcuate summit benches, and satellite cones. Volcanic rocks dredged from nine of these seamounts are all depleted mid-ocean ridge basalt (N-MORB). Off-axis seamounts on the ridge flanks generally have primitive lavas with high Mg# (66-68) except for those dredged from calderas, which are less primitive (Mg# 63-65). In contrast, cones on the floor of the axial rift valley have Mg# 62-65. In general, off-axis seamount lavas are more primitive than axial lavas. Major element chemistry also suggests that the magmas of off-axis seamounts are produced at higher pressure and with less melting than axial cone lavas. The slow to moderate spreading Mid-Atlantic Ridge at 26°S is not affected by hot spots. Its seamounts and cones have shapes, size distribution, abundance, and chemistry similar to seamounts near the fast-spreading East Pacific Rise. If these results are representative for the South Atlantic and other slow-spreading ridge crests, then near-axis, non-hotspot seamount production is not restricted to fast and medium spreading centers.

doi: 10.1086/629296pmid: N/A

Debris distribution and content in Spitsbergen icebergs varies, but basal debris-rich layers (often ~1 m-thick) are the most important sources of iceberg rafted material. Angular iceberg rafted clasts in the glaci-marine record imply the presence of nunataks or valley walls, rather than unbroken ice sheet surfaces, indicating paleo-ice mass configuration. Calculated iceberg melt rates imply that most iceberg rafted debris is released within Spitsbergen fjords. Sedimentation from icebergs is calculated as 1 mm $$a^{-1}$$ for inner Mollerfjorden, and 8 mm $$a^{-1}$$ for inner Kongsfjorden, an order of magnitude lower than total sediment inputs. Turbid subglacial meltwater is a more important debris source. A simple model is proposed to calculate the flux of basal debris in icebergs and iceberg sedimentation rates.

doi: 10.1086/629297pmid: N/A

Ion probe U-Pb zircon analyses yield a crystallization age of $$74 \pm 3 Ma$$ for both peraluminous and metaluminous plutons within the Old Woman-Piute batholith. These analyses reveal that about 50% of the zircons from the peraluminous Sweetwater Wash granite are restitic. Inherited zircons from this batholith yield ages ranging from older than 1800 Ma to ~1100 Ma, and most are similar in age to Proterozoic igneous rocks in the eastern Mojave Desert. $$^{40}Ar/^{39}Ar$$ hornblende ages from the metaluminous granodiorite are $$73 \pm 2 Ma$$, and mica ages from both plutonic series are $$70 \pm 2 Ma$$. K-feldspar age spectra yield age gradients with initial steps between 60 and 67 Ma and rise to plateau ages of $$70 \pm 2 Ma$$. Several K-feldspars from the granodiorite yield elevated "plateau" ages after 50% $$^{39}Ar$$ released that result from excess argon. Uplift rates as high as 4 mm/yr between 74 and 70 Ma are required by the narrow age range for all minerals.