Southwest New Research Articles

Persistence and Stability of Fish Community Structure in a Southwest New York Stream

-We used multivariate and nonparametric statistics to examine persistence and stability of fish species in the upper 43 km of French Creek, New York. Species occurred in upstream and downstream groups in 1937 that persisted in 1979. However, the downstream group expanded its range in the drainage from 1937 to 1979 at the expense of the upstream group. A dam prevented further upstream expansion of the downstream group. Ranks of species abundances were stable, as tests of group similarity were significant. The abundances and distributions of benthic species were stable across seven sampling dates in 1980 despite several floods and repeated removals by sampling that could have altered community structure. We conclude that the fish community in French Creek persisted and was stable over the 42-yr interval, 1937-1979, and that abundances of benthic species were stable in summer 1980.

A comparison of high mountain lakes at different altitudes in southwest New Zealand

A comparison of high mountain lakes at different altitudes in southwest New Zealand

Asmat art: woodcarvings of southwest New Guinea

Asmat Art features the world-renowned woodcarvings of the Asmat, former head-hunters who live in the western half of New Guinea. Working with stone adzes, bone, and bits of shell, early carvers fashioned the local wood into stunning masterpieces. Editor Dirk Smidt has gathered together distinguished experts on Asmat art and culture to describe the physical and spiritual world of the Asmat, the techniques and motifs used by master carvers, and to present an historical overview of European collections of Asmat art.

A vegetation switch as the cause of a forest/mire ecotone in New Zealand

Abstract. Vegetation switches are those processes in which there is positive‐feedback between vegetation and environment, i.e. a vegetation state modifies its environment producing conditions more favourable to itself (Wilson & Agnew 1992). Switches can produce and maintain abrupt ecotones between plant communities.Such a sharp ecotone exists between beech‐podocarp forest and mire vegetation, both on deep peat, in southwest New Zealand. One such site was examined. There was no apparent explanation for the ecotone in the present topography nor in the substrate.Levelled transects through the forest demonstrated that most seedling establishment occurred on dead fallen tree boles. These microsites were significantly richer in N, P and K than the wet sump microsites. We argue that this is a mechanism whereby trees can become established in the forest, but not in the open mire. In the forest, the presence of trees ensures the presence of dead‐log microsites on the ground, permitting tree seedlings to grow. In the mire, there are no such micro‐sites, and trees cannot establish.The ecotone may be sharpened because of the presence of an ecotonal band of the small tree Leptospermum sco‐parium between forest and mire. This species can reproduce vegetatively by root suckers in the mire. Its boles are light, and even if they fall to the mire surface they are not thick enough to form a substrate for tree‐seedling establishment. The larger tree species may be prevented from falling onto the mire by the wind‐sheltering of the forest, and by the zone of Leptospermum.The postulated process would represent a new kind of water‐/nutrient‐mediated switch, of Type 1 (‘One‐sided’). It may occur in many waterlogged forests worldwide.

Compositional gradients in large reservoirs of silicic magma as evidenced by ignimbrites versus Taylor Creek Rhyolite lava domes

The Taylor Creek Rhyolite of southwest New Mexico consists of 20 lava domes and flows that were emplaced during a period of a few thousand years or less in late Oligocene time. Including genetically associated pyroclastic deposits, which are about as voluminous as the lava domes and flows, the Taylor Creek Rhyolite represents roughly 100 km3 of magma erupted from vents distributed throughout an area of several hundred square kilometers. Major-element composition is metaluminous to weakly peraluminous high-silica rhyolite and is nearly constant throughout the lava field. The magma reservoir for the Taylor Creek Rhyolite was vertically zoned in trace elements, 87Sr/86Sr, and phenocryst abundance and size. Mean trace-element concentrations, ranges in concentrations, and element-pair correlations are similar to many subalkaline silicic ignimbrites. However, the polarity of the zonation was opposite that in reservoirs for ignimbrites, for most constituents. For example, compared to the Bishop Tuff, only 87Sr/86Sr and Sc increased upward in both reservoirs. Quite likely, a dominant but nonerupted volume of the magma reservoir for the Taylor Creek Rhyolite was zoned like that for the Bishop Tuff, whereas an erupted, few-hundred-meter-thick cap on the magma body was variably contaminated by roof rocks whose contribution to this part of the magma system moderated relatively extreme trace-element concentrations of uncontaminated Taylor Creek Rhyolite but did not change the sense of correlation for most element pairs. The contaminant probably was a Precambrian rock of broadly granitic composition and with very high 87Sr/86Sr. Although examples apparently are not yet reported in the literature, evidence for a similar thin contaminated cap on reservoirs for large-volume silicic ignimbrites may exist in the bottom few meters of ignimbrites or perhaps only in the pumice fallout that normally immediately precedes ignimbrite emplacement. 87Sr/86Sr in sanidine phenocrysts of the Taylor Creek Rhyolite is higher than that of their host whole rocks. Covariation of this isotope ratio with sanidine abundance and size indicates positive correlations for all three features with decreasing distance to the roof of the magma reservoir. The sanidine probably is more radiogenic than host whole rock because growing phenocrysts partly incorporated Sr from the first partial melt of roof rocks, which contained the highly radiogenic Sr of Precambrian biotite ± hornblende, whereas diffusion was too slow for sanidine to incorporate much of the Sr from subsequently produced less radiogenic partial melt of roof rocks, before eruption quenched the magma system. Disequilibrium between feldspar phenocrysts and host groundmass is fairly common for ignimbrites, and a process of contamination similar to that for the Taylor Creek Rhyolite may help explain some of these situations.

Community-based exercise and weight control: diabetes risk reduction and glycemic control in Zuni Indians

Cardiovascular disease is a significant health problem for the Zuni Indians of southwest New Mexico, in part because of high rates of non-insulin-dependent diabetes mellitus (NIDDM). The Zuni Diabetes Project was initiated in July 1983 to reduce rates of obesity and provide primary and secondary prevention of NIDDM. Two studies of the project's activities have been carried out to date. After 2 y of follow-up, diabetic participants in an exercise program compared with diabetic nonparticipants experienced weight loss, a drop in fasting blood glucose values, and reductions in the use of hypoglycemic medications. In a weight-loss competition, 45% (122/271) of the enrollees finished and lost greater than or equal to 2.3 kg. The results of these two studies demonstrate that 1) participation in a community-based exercise program can produce significant weight loss and improvement in glycemic control in Zuni Indians with NIDDM and 2) weight-loss competitions appear to be an important public health model for health-behavior change in communities similar to that of Zuni, NM.

Paleomagnetism of Latest Cambrian‐Early Ordovician and Latest Cretaceous‐Early Tertiary rocks of the Florida Mountains, southwest New Mexico

Paleomagnetic investigations have been carried out on latest Cambrian to Early Ordovician alkaline intrusions, their contact intermediate to mafic composition metaigneous rocks, and overlying lower Paleozoic and early Tertiary strata in the Florida Mountains, southwestern New Mexico. A well‐defined, predominantly normal polarity magnetization of high median destructive induction and high, usually discrete laboratory unblocking temperatures is isolated from sites in metaigneous rocks in contact with syenites exposed on the northwestern flank of this late Cenozoic uplift. Combinations of stable endpoint and planar data are used in data analysis at the site level (D = 262°, I = −30°, α95 = 10°, k = 12; N = 22 site means). In addition to the presence of reverse polarity magnetizations at two sites, the dispersion of site mean directions in these rocks (VGP angular standard deviation of 19.6°) suggests that considerable variation of the geomagnetic field has been averaged. After correction for a combination of early Tertiary (Laramide) deformation and late Tertiary Basin‐Range block tilting, this magnetization gives a paleomagnetic pole at 6°S, 169°E and is interpreted to have been acquired soon after intrusion of alkaline plutons at 503±10 Ma, based on published U‐Pb zircon isotopic age determinations. A lack of well‐defined, coherent magnetizations in the plutonic rocks may reflect the complex subsolidus alteration history of these rocks, as interpreted with Rb‐Sr isotopic data. Hematitic sandstones of the earliest Ordovician Bliss Formation, which directly overlie the crystalline rocks in portions of the range, give dual‐polarity magnetizations whose directions suggest that these rocks have probably been remagnetized; this remagnetization possibly occurred during deformation in late Cretaceous/Tertiary time. Ordovician carbonate rocks of the El Paso and Montoya formations give only a Holocene magnetization. Paleocene to earliest Eocene(?) hematite‐cemented clastic rocks of the Lobo Formation possess dual‐polarity magnetizations. The origin of their magnetization is difficult to assess with the available data. In total, however, paleomagnetic data from the Bliss and Lobo formations argue against any significant vertical axis rotation of the area since Laramide deformation ending in Early Tertiary time. The intermediate to mafic composition metaigneous rocks from the Florida Mountains provide a latest Cambrian‐Early Ordovician paleomagnetic pole that compares favorably with other North American paleopoles satisfying several acceptance criteria. The Florida Mountains data further support recent constructions of apparent polar wander paths for North America and thus the hypothesis that significant counterclockwise rotation of North America occurred during the Ordovician.

Mid‐Paleozoic age of granitoids in enclaves within Early Cretaceous granulites, Fiordland, southwest New Zealand

Abstract Orthogneisses of granite, quartz monzonite, monzonite, and tonalite, collectively termed granitoid, occur locally as isolated enclaves within the Early Cretaceous granulite terrain (Western Fiordland Orthogneiss—WFO) in Fiordland, southwest New Zealand. Discordant U‐Pb zircon isotopic data (7 fractions) from four granitoid samples from enclaves at George Sound, define an upper intercept age of 341 ± 34 Ma that is interpreted as approximating the time of formation of the granitoid suite. The lower intercept age of 93 ± 37 Ma is interpreted as approximating the time of zircon isotopic disturbance by major episodic Pb loss. Isotopic disturbance probably accompanied high‐temperature conditions at the time of incorporation of the granitoid rocks into the WFO protolith, and during subsequent granulite meta‐morphism; both occurred in the interval c. 120–110 Ma. Five whole‐rock samples from enclaves at George Sound yield an apparent Rb‐Sr isochron age of 391 ± 48 Ma, in agreement with the zircon data, an...

Zoned garnets in metapelites in western Fiordland, southwest New Zealand: Polychronic crystallisation and insight into the nature and extent of Early Cretaceous regional metamorphism

Abstract Two distinct types of compositional zoning are recognised in idioblastic to subidioblastic garnets in amphi‐bolite facies metapelites in western Fiordland. One type occurs in kyanite (± sillimanite)‐bearing rocks and is characterised by a core to rim increase in CaO content, and a sympathetic decrease in FeO and MgO, with MnO remaining essentially constant, or decreasing slightly near the rim. This zoning pattern formed during garnet growth, and records adjustment of mineral equilibria under conditions of increasing load pressure. Garnets showing the same zoning pattern occur within metapelitic rocks in rafts within nearby Western Fiordland Orthogneiss (equivalent to western Fiordland granulites), and within a contamination zone overprinting this orthogneiss at the margin of a highly aluminous raft. Garnet zoning thus provides evidence that these rocks experienced the same Early Cretaceous tectono‐metamorphic event. The pressure increase has been attributed to crustal thickening by tectonic loadi...

Magnetic polarity stratigraphy of Pliocene and Pleistocene fossil vertebrate localities in southeastern Australia

Research Article| November 01, 1991 Magnetic polarity stratigraphy of Pliocene and Pleistocene fossil vertebrate localities in southeastern Australia MICHAEL J. WHITELAW MICHAEL J. WHITELAW 1Department of Geological Sciences, University of Texas at El Paso, El Paso, Texas 79968-0555 Search for other works by this author on: GSW Google Scholar Author and Article Information MICHAEL J. WHITELAW 1Department of Geological Sciences, University of Texas at El Paso, El Paso, Texas 79968-0555 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1991) 103 (11): 1493–1503. https://doi.org/10.1130/0016-7606(1991)103<1493:MPSOPA>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation MICHAEL J. WHITELAW; Magnetic polarity stratigraphy of Pliocene and Pleistocene fossil vertebrate localities in southeastern Australia. GSA Bulletin 1991;; 103 (11): 1493–1503. doi: https://doi.org/10.1130/0016-7606(1991)103<1493:MPSOPA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A series of magnetic polarity stratigraphy studies are presented from localities in Victoria and southwest New South Wales, Australia. These localities were examined in order to improve the age resolution of Pliocene and Pleistocene fossil vertebrate sites in south-eastern Australia. The following age constraints should now be used for these southeastern Australian local faunas: For-syth's Bank, Kalimnan Stage and >4.42 Ma; Hamilton, Gilbert Chron (4.35-4.42 Ma); Boxlea, Gilbert Chron? (>4.06 Ma); Parwan, Gilbert Chron (4.06-4.20 Ma); Coimadai, Late Gilbert Chron (3.35-3.82 Ma); Fisher-man's Cliff, Gauss Chron (2.42-2.87 Ma); Bone Gulch, Early Matuyama Chron (1.82-2.42 Ma); Dog Rocks, Early Matuyama Chron (1.82-2.42 Ma); Duck Ponds, Late Matuyama Chron (>0.71 Ma and probably <1.63 Ma); Limeburner's Point, Late Matuyama or Brunhes Chron (<0.96 Ma); Hines Quarry, Late Matuyama or Brunhes Chron (<0.96 Ma); and Nelson Bay, Late Matuyama Chron (0.71-1.63 Ma). This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Incompletely reset RbSr systems from a Cambrian red-rock granophyre terrane, Florida Mountains, New Mexico, U.S.A.

A zircon separate from a syenite from the Florida Mountains, near Deming, southwest New Mexico, yields a 514 ± 3-Ma concordia upper-intercept age. However, RbSr isochron dates from syenite and other igneous core rocks in the Florida Mountains range from ∼ 519 to ∼ 381 Ma. The RbSr dates are based on scattered linear arrays suggestive of open-system conditions for the rocks. Calculated initial 87 Sr 86 Sr ratios are highly variable for presumably coeval rocks, and some are anomalous values below 0.699. KAr dates from these igneous core rocks range from 555 to 418 Ma, while an andesite dike, possibly associated with Tertiary mineralization in the area, yields a date of 2.7.9 Ma. The zircon UPb concordia age can be reconciled with the available stratigraphic controls that require a pre-Middle-Ordovician age for some granite and syenite core-rock exposures. Collectively, the geochronologic data argue for Early Paleozoic plutonism at ∼ 510–550x Ma, with a resetting event affecting the RbSr and KAr systematics in the syenite and alkali granite in the Tertiary. The resetting mechanism may have involved Sr loss, possibly through ion-exchange reactions between feldspar and moderately heated fluids, as well as loss of Ar and/or addition of K in the syenites, alteration of the Th/U ratio in the syenite and granite, and production of brick-red K-feldspars due to the incorporation of minute hematitic inclusions. δ 18O-values for several whole-rock samples range between + 5.8 and + 9.2‰, consistent with a high water/rock ratio and alteration temperatures of ⪅ 300°C. World-wide, red-rock granophyre terranes commonly yield discordant RbSr and UPb dates, with RbSr dates scattered and systematically younger than UPb ages. We suggest that red-rock granophyres are inherently undatable by the RbSr technique because of Sr loss from K-feldspars during penetration of Fe-oxidizing solutions.

Petrology and petrogenesis of the Ministers Island dike, southwest New Brunswick, Canada

The Ministers Island dike in southwest New Brunswick is a quartz tholeiitic member of the Late Triassic to Early Jurassic dike suite that outcrops along the east coast of North America. The dike's phenocryst assemblage is orthopyroxene + augite + plagioclase. The combination of petrographic, chemical (both phase and whole rock), isotopic and experimental work on representative samples from the dike places important constraints on the petrogenesis of the Ministers Island dike and by analogy on the other members of the dike suite. The petrographic, analytical and experimental results show that the Ministers Island dike magma underwent high pressure (0.8 to 1.0 GPa) fractionation of augite, followed by augite + orthopyroxene, and finally augite + plagioclase. The absence of olivine as either a phenocryst or an experimentally observed high pressure liquidus phase indicates that the magma evolved away from the Ol-Cpx-Opx-Plag pseudo-invariant point while still at high pressure and there was sufficient time for any olivine to settle out of the magma prior to emplacement of the dike. The Sr and Nd isotopic results support a metasomatised mantle source similar to EMI but with slightly more radiogenic Nd.

The Taylor Creek Rhyolite of New Mexico: a rapidly emplaced field of lava domes and flows

The Tertiary Taylor Creek Rhyolite of southwest New Mexico comprises at least 20 lava domes and flows. Each of the lavas was erupted from its own vent, and the vents are distributed throughout a 20 km by 50 km area. The volume of the rhyolite and genetically associated pyroclastic deposits is at least 100 km3 (denserock equivalent). The rhyolite contains 15%–35% quartz, sanidine, plagioclase, ±biotite, ±hornblende phenocrysts. Quartz and sanidine account for about 98% of the phenocrysts and are present in roughly equal amounts. With rare exceptions, the groundmass consists of intergrowths of fine-grained silica and alkali feldspar. Whole-rock major-element composition varies little, and the rhyolite is metaluminous to weakly peraluminous; mean SiO2 content is about 77.5±0.3%. Similarly, major-element compositions of the two feldsparphenocryst species also are nearly constant. However, whole-rock concentrations of some trace-elements vary as much as several hundred percent. Initial radiometric age determinations, all K−Ar and fission track, suggest that the rhyolite lava field grew during a period of at least 2 m.y. Subsequent 40Ar/39Ar ages indicate that the period of growth was no more than 100 000 years. The time-space-composition relations thus suggest that the Taylor Creek Rhyolite was erupted from a single magma reservoir whose average width was at least 30 km, comparable in size to several penecontemporaneous nearby calderas. However, this rhyolite apparently is not related to a caldera structure. Possibly, the Taylor Creek Phyolite magma body never became sufficiently volatile rich to produce a large-volume pyroclastic eruption and associated caldera collapse, but instead leaked repeatedly to feed many relatively small domes and flows.

Venom characteristics as an indicator of hybridization between Crotalus viridis viridis and Crotalus scutulatus scutulatus in New Mexico

One hundred and thirteen venoms from 46 populations of Crotalus viridis viridis were screened by immunodiffusion for protein toxins antigenically similar to the phospholipase A2 (PLA) toxin 'Mojave toxin', using a polyclonal antibody to it's basic PLA subunit. Venom i.p. LD50 values in mice were recorded from 22 of the 46 populations. The venoms of three of 14 specimens from southwest (S.W.) New Mexico and one specimen from northern Arizona were immunologically positive by the immunodiffusion tests and produced low LD50 values (0.38-0.65 mg/kg) compared to all immunologically negative venoms (0.9-5.5 mg/kg). These four specimens were morphologically typical for C. v. viridis and their venoms were the only samples of 15 southern New Mexico specimens examined by reverse phase HPLC to exhibit peaks corresponding to the acidic and basic subunits of Mojave toxin. Alkaline polyacrylamide gel electrophoresis (PAGE) analysis of the recombined subunit peaks from the C.v. viridis venom from the S.W. New Mexico specimens showed more similarity to Mojave toxin from C.s. scutulatus venom than to similar toxins in C.v. concolor venom. The combined results of the immunodiffusion, lethal toxicity tests, HPLC profiles and PAGE analysis strongly suggest that the venoms of the three New Mexico specimens contain Mojave toxin(s), as a result of some previous hybridization with C.s. scutulatus. The northern Arizona specimen likely contains 'concolor toxin' through integration with C.v. concolor in its' genetic background.

Origin and metamorphic history of an Early Cretaceous polybaric granulite terrain, Fiordland, southwest New Zealand

Regionally extensive two-pyroxene granulites in Fiordland, southwest New Zealand, are products of metamorphism of a suite of anhydrous magmas which crystallized two pyroxenes. The granulite protolith (igneous charnockitic rock) synkinematically intruded metasediment and other orthogneiss in an Early Cretaceous subduction-related magmatic arc, and during cooling experienced deformation-induced recrystallization to form granoblastic gneiss. The granulites occur side by side with coeval rocks of amphibolite facies. Mineral zoning and textural relationships in both granulites and amphibolite facies rocks provide evidence of two distinct periods of crystallization: 1) an early high temperature, comparatively low pressure event accompanying magmatic intrusion (andalusite-sillimanite facies series recorded locally in the country rock), followed by 2) high pressure metamorphism under conditions of ∼650°–700° C at ∼12–13 kbar. Garnet granulite locally overprinted earlier formed two-pyroxene granulite during the latter event. The pressure increase (∼6 kbar) between the two events is attributed to crustal thickening by overthrusting, and is equivalent to unloading of a ∼20 km thick slab over rocks already buried at mid-crustal depths. Both events occurred over a < 20 m.y. interval, between the time of magmatic emplacement of the granulite protolith and uplift-controlled final cooling of the terrain. The Phanerozoic granulites in Fiordland share some petrologic similarities with Precambrian granulite terrains, suggesting that at least some aspects of the former may serve as a useful model for development of the latter.

Southwest New World Island stratigraphy: new fossil data, new implications for the history of the Central Mobile Belt, Newfoundland

The identification of fossils from 16 new localities on southwestern New World Island has resulted in the refinement and simplification of local stratigraphy and in the correlation of rock units across an intensely deformed area. Complex stratigraphic classifications of earlier workers are replaced by a simpler scheme consisting of four formations: the Summerford Formation (oldest), Rogers Cove Formation, Sansom Formation, and Goldson Formation. Mélange units are fault related and cannot be included in the stratigraphy. Fossil and lithological data provided here confirm previous indications that the rocks of the Summerford Formation are the remains of a long-lived volcanic-island complex. This complex is overlain by a markedly diachronous coarsening-upwards sequence of marine clastic sediments, now represented by the Rogers Cove, Sansom, and Goldson formations.The new stratigraphic data, combined with structural evidence, indicate that the stratigraphy is repeated across several bedding-parallel faults. Significant diachroneity of formation boundaries and the absence of syndepositional high-angle faults suggest deposition in one large mid-Paleozoic basin rather than in a number of small dynamic basins.

Cretaceous geotectonic patterns in the New Zealand Region

In the mid‐Cretaceous, around 105±5 Ma, the tectonic regime in New Zealand changed significantly. From the Permian to the Early Cretaceous most rocks formed under the influence of convergent margin tectonics and comprise incomplete remnants of magmatic arcs, forearc basins, trench slope basins, and accretionary complexes. This tectonic pattern was ended by the approach and collision of the spreading ridge between the Phoenix and Pacific plates. Crustal extension, leading eventually to fragmentation of the continental crust, commenced immediately after ridge collision. Precollisional and syncollisional events deformed and thickened the accretionary prism, with concurrent folding and thrusting of more arcward elements. Cretaceous granulites in southwest New Zealand are thought to result from the closure of a small backarc basin or marginal sea slightly earlier than the main ridge collision. Postcollisional block faulting led to thick nonmarine sedimentation in new basins that cut across old terrane boundaries, except in the outboard area where a thick wedge of marine rocks was deposited over the subsiding accretionary prism. After oblique ridge trench collision along the New Zealand margin, the Phoenix‐Pacific Ridge propagated to the southwest to link with zones of incipient spreading in the Tasman Sea and south of Australia. Subsequently, the eastern part of the ridge continued to migrate south with increasing offset along the Udinsev Fracture Zone. In the Late Cretaceous a duplicate ridge developed. The southern branch continued to move southward and collided with the trench west of the Antarctic Peninsula in the Cenozoic. The northern branch is the extant East Pacific Ridge.

Early Cretaceous Vein-Related Garnet Granulite in Fiordland, Southwest New Zealand: A Case for Infiltration of Mantle-Derived CO2-Rich Fluids

Regionally extensive two-pyroxene granulite facies orthogneisses of Early Cretaceous age in Fiordland, southwest New Zealand, are criss-crossed by garnet-bearing feldspathic veins (and dikes) having associated marginal reaction zones of garnet granulite. The two-pyroxene granulites resulted from fluid-absent meta-morphism of a suite of synkinematic primary anhydrous intrusions. Subsequent restricted formation of garnet granulite in feldspathic compositions, and locally eclogite in ultramafic compositions, proceeded chiefly via reactions involving hornblende breakdown, and occurred in response to sharply increased load pressure and local lowering of water activity. The restricted occurrence of carbonate scapolite and -rich fluid inclusions in the vein areas suggests that water activity was lowered by infiltrating carbonic fluids. Infiltration occurred along pre-existing fracture systems, many of which were already filled with plagio-clase-rich veins and dikes. Published carbon isotope data for in scapolite within the veins indicates a mantle source. Open system behavior accompanying infiltration favored garnet stability by lowering bulk rock ferric/ferrous ratio and content. Fluid infiltration occurred at or near peak metamorphic pressure (~12 kbar at 650-700°C). Granulite metamorphism was of short duration (< 20 m.y.) and accompanied tectonic thickening in a subduction-related magmatic arc.

Sm-Nd and Rb-Sr isotopic and geochemical systematics in Phanerozoic granulites from Fiordland, southwest New Zealand

Sm-Nd and Rb-Sr isotopic analyses are reported for granulite facies orthogneisses from Fiordland southwest New Zealand. Whole-rock samples define a Rb-Sr isochron age of 120±15 Ma and an initial 87Sr/86Sr ratio of 0.70391±4. ɛNd values (at 120 Ma) show a relatively wide range of from −0.4 to 2.7 indicating decoupling of Sr-Nd isotope systems. Associated ultramafic rocks have initial 87Sr/86Sr ratios of from 0.70380 to 0.70430 and ɛNd values of from 0.1 to 3.0. The different initial ratios suggest that the various intrusions, although contemporaneous, were not derived through fractionation of a single parent magma. A metasedimentary enclave incorporated during emplacement of the granulitic rocks preserves a Proterozoic isotopic signature with a measured ɛNd(0) value of −10.2, 87Sr/86Sr ratio of 0.73679 and a TNd provenance age of 1490 Ma. The Rb-Sr whole rock age of the granulites is the same as obtained from recent U-Pb zircon dating (Mattinson et al. 1986) and is interpreted as the time of magmatic emplacement and essentially contemporaneous granulite facies metamorphism. Rb-Sr and Sm-Nd analyses of mineral systems indicate that the terrain had cooled below ∼300° C by ∼100 Ma providing further evidence that high grade metamorphism was of exceptionally short duration.

ANOMALOUS WET AND DRY SPELLS IN THE CENTRAL UNITED STATES, 1931–75

The Palmer Index (PI) was employed to compare anomalous dry spells with anomalous wet spells for 76 climatic divisions in the central United States. Comparisons were made in terms of the frequency, severity and persistence of wet and dry climatic episodes for the period 1931–75. The results indicated more frequent dry anomalies in the southwest (New Mexico and west Texas) portion of the study area and more frequent wet anomalies in the northeast (Illinois and Iowa). Not surprisingly, these are the most arid and most humid portions respectively. The results of the analysis for persistence were less clear. Typically dry anomalies lasted longer than wet anomalies, especially as the minimum length was increased. This was especially apparent when comparing the very longest dry spell with the very longest wet spell during the 45-year study period. Thus there are more but shorter wet spells and less frequent, but typically longer lasting dry spells. This final relationship posits the question of feedback process...