Mineralogic and Oxygen Isotopic Studies of Open System Magmatie Processes in the South Kawishiwi Intrusion, Spruce Road Area, Duluth Complex, Minnesota

The South Kawishiwi intrusion, located along the western marginof the Duluth Complex, Minnesota, is one of several compositeintrusions that are found in the Complex. The Duluth Complexis the principal exposed plutonic portion of the 1.1 Ga MidcontineniRift system. In the Spruce Road area the South Kawishiwi intrusionis divided into seven distinct units that are part of the broaderSouth Kawishiwi Troctolite Series defined by Severson (Tech.Rep. NRRI/TR-91/13a, Natural Resources Research Institute, Universityof Minnesota, Duluth, 1994). Units may be characterized as follows:Unit I—basal accumulation of heterogeneous gabbro, troctolite,and norite; Unit II—norite with abundant inverted pigeonite;Unit III—troctolite and olivine gabbro with local oxide-richlayers; Unit IV—mlatroctolite, troctolite, olivine gabbro;Unit VI—increased plagioclase abundance in troctolitesand leucocratic troctolites; Unit VI—strongly alteredtroctolite; Unit VII—similar to Unit V, troctolite andleucocratic troctolite. Country rocks in the Spruce Road areaare granodiorite to quartz monzonite of the Archean Giants RangeBatholith. Sutfide mineralization, consisting of 1–5 vol.% of disseminated pyrrhotitt, cubanite, chalcopyrite and pentlandite,occurs in Units I, II, III, and VI. Oxygen isotopic analysesindicate that Unit II has experienced extensive crustal contamination.¹⁸O values of Unit II range from 6.9 to 7.1% and are ¹⁸O enrichedcompared with values of 5.1–6.8% found in other units.Silica contamination is indicated based not only on ¹⁸O values,but also by the predominance of orthopyroxene in the unit. Possiblehigh-¹⁸O contaminant rocks include the Giants Range Batholithand pelitic rocks of the Lower Proterozoic Virginia Formationor Biwabik Iron Formation. Mass balance computations suggestthat units in the Spruce Road area may be related through varyingdegrees of fractionation of a high-Al, olivine tholeiite magma.Modeling of trace element concentrations and variations in mineralchemistry suggest that discontinuities within the major unitsdeveloped by in situ boundary-layer equilibrium crystallizationof solidification zones 20–50 m in thickness, followedby recharge of fresh magma. Upward enrichment of incompatibleelements, olivine Fa content, and plagioclase Ab content maybe effectively explained by this process. ¹⁸O values of uncontaminatedrock types are strongly correlative with modal mineralogy, andcan also be modeled by boundary-layer fractionation, A parentalmagma ¹⁸ O value of 6.3% is calculated for Unit VII based onolivine and plagioclase values, and is similar to that of severalother large, layered mafic intrusives. KEY WORDS: Duluth Complex; South Kawishiwi Intrusion; high-Al olivine tholeiite; open system crystallization; oxygen isotopes ^* Present address: Korea Basic Science Center, Isotope Research Group, Yeocun Dong 224–1, Yusung Ku, Yusung P.O. Box 41, Taejean 305–333, Korea     

Petrology of Biotite-Cordierite-Garnet Gneiss of the McCullough Range, Nevada II. P-T-aH2O Path and Growth of Cordierite During Late Stages of Low-P Granulite-Grade Metamorphism

Thermodynamic calculations based on addition of mass balanceequations to the Gibbs Method (Spear, 1986) are used to modelthe cordierite-producing reaction in pelitic gneiss from theMcCullough Range, southern Nevada. Calculations which treatthe model paragenesis as a system open to transfer of H₂O areconsistent with textural relations. Results indicate that cordieritegrew by the continuous net-transfer reaction: 0?76 BIO+1?72 SILL+3? 55 QTZ+0?27 PLG+0?005 GRT +0?06Al₂R²⁺_–1Si_–1[BIO]1?02 KSP+0?76 H₂O +0?30 FeMg_–1[CRD]+0?15FeMg_–1[BIO]+0?0005 FeMg_–1[GRT] +0?005 CaNaAl_–1Si_–1[PLG] with decreasing P, decreasing T, and increasing a_H2O The steepretrograde dP/dT path for these low-pressure granulites contrastswith isobaric cooling paths typical of higher pressure granulites,and suggests uplift and erosion were active during Proterozoicgranulite-grade metamorphism in this area.     

Cosmogenic 10Be exposure age dating across Early to Late Weichselian ice-marginal zones in northwestern Russia

Rock samples from the Kanin Peninsula and the Timan Ridge were analysed for in situ cosmogenic ¹⁰Be for exposure age dating purposes. Crystalline rocks were sampled at four sites on the Kanin Peninsula, either from bedrock outcrops or from glacial erratics, giving overall similar ¹⁰Be ages. Outcropping sandstone and crystalline erratics were available from three sites at the Timan Ridge. The highly weathered sandstone gives substantially younger ¹⁰Be ages than the adjacent erratics. The exposure ages from the Kanin Peninsula suggest that the last deglaciation of this area took place between 55 and 37 ¹⁰Be kyr ago, in agreement with a preceding Kara Sea glaciation (55-45 kyr BP). The northwest coast of the peninsula was probably just outside the maximum limit of the last Scandinavian glaciation (20-17 kyr BP). Glacial erratic exposure ages from the Timan Ridge suggest that the 55-45 kyr BP Kara Sea glaciation reached the northern part of the ridge. The exposure dates do not show conclusive evidence regarding the existence of a Timan Ridge ice cap.     

WILBUR AND ME

ABSTRACT. Wilbur Zelinsky has spent his spectacular scholarly career enhancing our understanding of American life through explorations into the nation's cultural geography. Although he has illuminated grand themes, he is also celebrated for his eclectic work on the geography of Americana. Few, however, may appreciate his lifelong enjoyment of American literature. Early in my scholarly journey, Wilbur encouraged my awakening interest in reading widely, beyond the boundaries of professional literature. Conversations with him about shared literary enthusiasms affirmed my reading proclivities. From classic novels to those of minor authors, commentary by journalists, biographies, and mass‐market fiction, literature offers provocative insights into American life and landscape. Reading widely takes one in unforeseen professional directions and yields unexpected rewards. Wilbur's sheer delight with the phenomena of the American scene, however seemingly trivial, instructs us to not overlook their possible geographical significance.     

Near-solidus Melting of the Shallow Upper Mantle: Partial Melting Experiments on Depleted Peridotite

We present the results of melting experiments on a moderatelydepleted peridotite composition (DMM1) at 10 kbar and 1250–1390°C.Specially designed experiments demonstrate that liquids extractedinto aggregates of vitreous carbon spheres maintained chemicalcontact with the bulk charge down to melt fractions of     

HYDROLOGICAL RESPONSE OF SMALL WATERSHEDS FOLLOWING THE SOUTHERN CALIFORNIA PAINTED CAVE FIRE OF JUNE 1990

Following the Painted Cave Fire of 25 June 1990 in Santa Barbara, California which burned 1214 ha, an emergency watershed protection plan was implemented consisting of stream clearing, grade stabilizers and construction of debris basins. Research was initiated focusing on hydrological response and channel morphology changes on two branches of Maria Ygnacio Creek, the main drainage of the burned area. Research results support the hypothesis that the response of small drainage basins in chaparral ecosystems to wildfire is complex and flushing of sediment by fluvial processes is more likely than by high magnitude debris flows. During the winter of 1990–1991, 35–66 cm of rainfall and intensities up to 10 cm per hour for a five-minute period were recorded with a seasonal total of 100% of average (normal) rainfall (average=63 cm/year). During the winter of 1991–1992, 48–74 cm of rainfall and intensities up to 8 cm per hour were recorded with a seasonal total of 115% of normal. Even though there was moderate rainfall on barren, saturated soils, no major debris flows occurred in burned areas. The winter of 1992–1993 recorded total precipitation of about 170% of normal, annual average intensities were relatively low and again no debris flows were observed. The response to winter storms in the first three years following the fire was a moderate but spectacular flushing of sediment, most of which was derived from the hillslopes upstream of the debris basins. The first significant storm and stream flow of the 1990–1991 winter was transport-limited resulting in large volumes of sediment being deposited in the channel of Maria Ygnacio Creek; the second storm and stream flow was sediment-limited and the channel scoured. Debris basins trapped about 23 000 m³, the majority coming from the storm of 17–20 March 1991. Sediment transported downstream during the three winters following the fire and not trapped in the debris basins was eventually flushed to the estuarine reaches of the creeks below the burn area, where approximately 108 000 m³ accumulated. Changes in stream morphology following the fire were dramatic as pools filled with sediment which greatly smoothed longitudinal and cross-sectional profiles. Major changes in channel morphology occur following a fire as sediment derived from the hillslope is temporarily stored in channels within the burned area. However, this sediment may quickly move downstream of the burned region, where it may accumulate reducing channel capacity and increasing the flood hazard. Ecological consequences of wildfire to the riparian zone of streams in the chaparral environment are virtually unknown, but must be significant as the majority of sediment (particularly gravel necessary for fish and other aquatic organisms) entering the system does so in response to fires. © 1997 John Wiley & Sons, Ltd.     

Genesis of hummocks in glaciofluvial corridors near the Keewatin Ice Divide, Canada

Glaciofluvial corridor hummocks (GCHs) within the Walker Lake map area, Canada, were examined in order to determine the character and genesis of these geomorphic features and their associated deposits. Located south of the Chantrey Moraine and north of the Keewatin Ice Divide, these corridors occur within a belt extending approximately 120 km east–west and approximately 60 km north–south. They are spaced 5–10 km apart and are hundreds of metres to several kilometres in width. They have undulating longitudinal profiles, abrupt material boundaries with the surrounding till and occur in valleys and over interfluves. Hummocks were investigated using longitudinal and perpendicular ground-penetrating radar (GPR) surveys in conjunction with pit excavations. From these analyses, the hummocks comprise a single lithofacies consisting of coarsely stratified, matrix-supported gravely sand to a depth of approximately 10 m. This sediment is similar to that of a 'sliding bed facies' observed in esker sediments and hyperconcentrated flow deposits, both of which are attributed to high meltwater discharges. Therefore, we hypothesize that the Walker Lake GCHs formed from sedimentation in cavities at the base of the ice sheet by a rapid influx of meltwater.     

From Code to Conduct. Professional Ethics in New Zealand Geography

There has been a major resurgence of interest in ethical matters among geographers during the past 3–4 years, both in New Zealand and internationally. This paper outlines some of the important arguments for behaving ethically in geographical research. It then argues that, rather than adopting a Code of Ethics in isolation, serious consideration needs to be given to the business of teaching ethics. The paper proposes a two-part teaching-and-learning strategy - the first part provides students with written and lecture material on ethics, and the second section presents real life case studies of ethical dilemmas confronted by geographers, which are used for detailed discussion in tutorial meetings.