Deuteric Alteration Research Articles

Chemical effects of deuteric alteration in some Kenyan trachyte lavas

AbstractThe Kenyan trachyte volcanoes Kilombe and Londiani show some unusual chemical features. Analyses of elements normally regarded as residual in salic peralkaline rocks do not show the linear covariance characteristic of these elements in other East African volcanoes. The scatter from expected trends is greatest for Y and the lanthanides, less for Nb and Rb, and not apparent for Zr. The lavas also show variable loss of Na, P, and Fe. It is suggested that this chemical deviation is the result of a deuteric event during the later stages of crystallization of the lavas. The flows showing the chemical deviations, in particular negative Ce anomalies, also tend to have their ferroaugite phenocrysts deuterically altered to a mineraloid closely resembling iddingsite.During the crystallization of these trachyte lavas an H2O-, Cl-, and F-rich vapour separates from the liquid. This absorbs some elements, particularly Na, P, Fe, Y, and the lanthanides from the silicate liquid as water-soluble or volatile halides. Some of the absorbed material is redeposited in other parts of the flow and the rest is lost altogether as the vapour escapes from the surface.

U-Th-Pb systematics in hydrothermally altered granites from the Granite Mountains, Wyoming

U-Th-Pb systematics were investigated in 15 samples representing two types of deuterically altered Archean granite (albitized and silicified-epidotized granite) from the Granite Mountains, Wyoming. The loss of K-feldspar during both types of deuteric alteration was accompanied by an extreme reduction of Pb content from roughly 40 ppm to less than 12 ppm in the most altered samples. Nine of the 15 samples yield anomalously young whole-rock Pb-Pb and Th-Pb ages compared to concordia ages for zircons and to whole-rock Pb-Pb and Th-Pb ages for samples of unaltered granite. The young ages are interpreted to be the result of radiogenic Pb loss during a middle Proterozoic metamorphism that disturbed several isotopic systems in the unaltered granite. The loss of radiogenic Pb from the whole-rock systems of the deuterically altered granites is most likely due to the absence of microcline. In many granitic rocks, potassium feldspar tends to act as a receptor for Pb that has been mobilized, and this effect may account for the closed-system behavior of Pb in whole-rock samples of the unaltered granite. It may also account for the apparent gain of radiogenic Pb during the Proterozoic by one sample which was collected at the edge of an alteration zone. The deuterically altered granites may have also lost U during the Proterozoic, but, except for two samples, the dominant U loss occurred relatively recently, probably during Tertiary uplift and erosion. The low common-Pb contents and rather high U and Th contents of the hydrothermally altered granites might seem to indicate that these rocks are well suited for geochronologic studies in the U-Th-Pb system because such rocks should have a high percentage of radiogenic Pb. Unfortunately, open-system behavior in response to post-crystallization metamorphism shows that apparent ages for these types of rocks must be interpreted with caution.

Nepheline-bearing rocks from the Poohbah Lake complex, Ontario: Malignites and Malignites

Malignites from the Poohbah Lake complex of northwestern Ontario, Canada are melanocratic cumulates. Cumulus pyroxene and apatite are poikilitically enclosed in a groundmass of large plates of intercumulus orthoclase and nepheline. Nepheline-feldspar fingerprint-like intergrowths occur. Nephelines are commonly zeolitized and pyroxenes altered to aggregates of biotite and/or garnet by deuteric alteration. Pyroxenes are weakly zoned from Di71 Hd18Ac11 to Di63Hd22Ac15, and are similar to the least evolved pyroxenes of other alkaline rocks. Nephelines all have compositions within the Morozewicz-Buerger convergence field and feldspars have a limited compositional range from Or88 to Or95. Perthites are absent.

Magnetic properties of dredged oceanic gabbros and the source of marine magnetic anomalies

Summary. Magnetic property studies (natural remanent magnetization, initial susceptibility, progressive alternating field demagnetization and magnetic mineralogy of selected samples) were completed on 45 samples of gabbro and metagabbro recovered from 14 North Atlantic ocean-floor localities. The samples are medium to coarse-grained gabbro and metagabbro which exhibit subophitic intergranular to hypidiomorphic granular igneous textures. The igneous mineralogy is characterized by abundant plagioclase, varying amounts of clinopyroxene and hornblende, and lesser amounts of magnetite, ilmenite and sphene. Metamorphic minerals (actinolite, chlorite, epidote and fine-grained alteration products) occur in varying amounts as replacement products or vein material. The opaque mineralogy is dominated by magnetite and ilmenite. The magnetite typically exhibits a trellis of exsolution-oxidation ilmenite lamellae that appears to have formed during deuteric alteration. The NRM intensities of the gabbros range over three orders of magnitude and give a geometric mean of 2.8 x 10-4gau~~ and an arithmetic mean of 8.8 x 10-4gauss. The Konigsberger ratio, a measure of the relative importance of remanent to induce magnetization, is greater than unity for the majority of the samples and indicates that remanent magnetization on average dominates the total magnetization of oceanic gabbros in the Earth's magnetic field. The magnetic properties of fresh and metamorphosed gabbros appeared to be similar. The majority of gabbros studied were characterized by median destructive fields greater than 200 Oe. The high stability is attributed largely to the effective subdivision of the magnetite grains by the

3-Gyr-old stromatolites from South Africa

WE report here a new record of early Precambrian stromatolites from the 3.0-Gyr Pongola Supergroup of South Africa (Fig. 1), which is exposed in parts of the southeastern Transvaal, Swaziland, northern Natal and Zululand. These rocks occur in an almost complete section found in the Wit-Mfolozi River valley, about 200km north of Durban. In that part of Natal the volcanic-–sedimentary Pongola Supergroup attains a maximum thickness of about 2,500 m. It is subdivided into two distinct north-east-dipping stratigraphic units; the lower 1,800-m thick Insuzi Group and the upper 700-m Mozaan Group1. Bedding and primary sedimentary structures are ubiquitously well preserved. Deformation is restricted to moderate cleaving of the argillaceous rocks. Evidence of regional metamorphism is restricted to incipient recrystallisation of the arenaceous and carbonate rocks; the lavas show evidence of deuteric alteration. Contact metamorphism is found where isolated diabase sills intrude the strata.

Mineralogy of intrusions in the Diablo Plateau, northern Trans-Pecos magmatic province, Texas and New Mexico

Electron probe microanalyses of minerals in the nine intrusive rock types of the Diablo Plateau document compositional changes accompanying magmatic differentiation. Plagioclase occurs only in the least differentiated rock type. Alkali feldspar phenocrysts show little compositional variation. Groundmass feldspar compositions were modified by exsolution and deuteric alteration, particularly during growth of natrolite. Nepheline compositions in these hypabyssal rocks are intermediate between those of volcanic and plutonic nephelines elsewhere. Mafic minerals show wider compositional variations. Clinopyroxene, the most abundant of these, changed from sodian augite toward hedenbergite, and then toward acmite. The more sodic pyroxenes crystallized later than feldspars and nepheline from residual liquids after emplacement at the now-exposed level. Amphiboles and biotites show progressive decreases in Ti, Mg, and tetrahedral Al/Si. In the least differentiated rock type and in some autoliths, the amphibole is kaersutite; in later formed rocks, hornblendes and sodic amphiboles are widespread. Other prominent minerals showing compositional variation are manganoan fayalite, aenigmatite, and eudialyte. For the nepheline syenites, crystallization temperatures are estimated at 700 to 725 °C, with log ( f O 2 ) = −20 and log ( f H 2 O ) = + 1.6 to 2.1.

Palaeomagnetism of marginal syenites and fractionated rocks of the Ilimaussaq intrusion, South Greenland

hagen, December, 20th 1976. https://doi.org/10.37570/bgsd-1976-25-11 Five sites (31 samples) from marginal augite syenites (<1175±9 m.y. and >1168±21 m.y.) of the Ilimaussaq peralkaline intrusion, South Greenland, give a mean direction of magne,. tisation D=253° , I = 70° and palaeomagnetic pole at 268°E, 38°N (a0, = 11°). The remanence is held by titanomagnetite; coercivity allld magnetic stability increase towards the mairgin of the intrusion and are associated with higher deuteric alteration states. The highly-unden;,aturated syenites in the interior of the intrusion (1168±21 m.y., 1150±28 m.y.) carry a weak ood partially stable remooence held by pyrrhotite; six sites (30 samples) give a mean direction of magnetisation D = 324°, I = 74° with a palaeomagnetic pole at 239°E, 73°N (a95 = 23°). The pole positions from this study correlate with poles derived from older Keweenawan rocks of North America; they show that the Great Logan palaeomagnetic loop (1300-1100 m.y.) is slightly larger than previously defined.

Precious metals in ocean-ridge basalts; implications for basalts as source rocks for gold mineralization

The Au, Ag, and S contents of the glass rims of tholeiitic pillow basalts dredged between 25 degrees N and 30 degrees N on the Mid-Atlantic Ridges are up to 7, 5, and 2.5 times greater, respectively, than those of the crystalline interiors of the basalts. Si, Fe, and Mn are also lower in the crystalline interiors relative to the glass rims, whereas K is higher. The differences in chemical composition between the rims and the interiors are attributed to loss of Au, Ag, S, Si, Fe, and Mn from the interiors of the basaltic pillows due to interaction of the hot basalt with sea water and gain in K by the interiors during subsequent low temperature interactions.Manganese crusts from an area of hydrothermal activity at 26 degrees N have exceptionally low Cu, Ni, Co, Fe contents and low Ir/Au and Ir/Ag ratios when compared to normal hydrogenous crusts that grow away from areas of submarine hydrothermal activity. Only a small fraction of the metals that are lost from the pillow interiors may be transported to the sea floor via the hydrothermal solutions that initially interact with the basalts; the remainder may reside within the basalt pile, for example, in interpillow regions.The Au contents of the fresh crystalline interiors of the basalts correlate with their Cr, Ni, and MgO contents. This suggests that Au in the basalt interiors is hosted by an early-crystallized phase such as Cr-spinel or olivine. The difference in Au content of the fresh interior (hosted by Cr-spinel and/or olivine) and that of the glass rim of the basalt is believed to represent that Au which would have been deposited on loosely bound sites (e.g., deuteric alteration products, grain boundaries, mesostasis phases, and, most important, sulfides within the pillow interior) had it not interacted at a high temperature with sea water.The Au and Ag contents of the glass rims vary by a factor of 10, confirming the existence of regional variations in the Au contents of basalts. The lowest Au and Ag contents in the glass rims are for basalts dredged from an area with no known hydrothermal activity.The results of this study suggest that basalts with high MgO, Cr, Ni, Cu, and Ir contents and low K 2 O and TiO 2 contents may constitute the most favorable type of source basalt. The study demonstrates that the metal available for ore generation in a source rock is not a function of the total metal content of the rock but rather a function of the mineralogical siting of the metal.

The Petrochemistry and Paleomagnetism of the Molson Dikes, Manitoba

Northeasterly trending, tholeiitic, low-alkali basaltic dikes which cut the Archean rocks of northeastern Manitoba have yielded a paleomagnetic pole at 114°W, 28°N, α95 = 12°. The material from nine sites was normally magnetized, and from one site, reversely magnetized. Potassium–argon age determinations suggest that the dikes were magnetized during late Aphebian. They may have been magnetized at the time of intrusion. Strongly differentiated dikes show intense and variable deuteric alteration, which is interpreted to be the cause of within-site dispersion of NRM. However, strong NRM is related to high contents of opaques regardless of the degree of alteration. Aphanitic dikes of low magnetite content and exhibiting minimal magmatic differentiation usually record moderate magnetic intensities and good groupings.

The Rb-Sr age of the Mashonaland dolerites of Rhodesia and its significance for palaeomagnetic correlation in Southern Africa

Abstract Palaeomagnetic correlation in southern Africa predicts that the age of the Mashonaland dolerites of Rhodesia is confined within the limits of the age of the Waterberg System of South Africa, viz., between 1,950 m.y. and about 1,750 m.y. Rb-Sr data from the dolerites confirm this prediction. One sample gave a mineral isochron of 1,850 ± 20 m.y., which may be the true age of emplacement and is certainly a reliable minimum estimate for it. Total rock samples from nine dykes define an isochron of age 1,910 ± 280 m.y. and initial 87Sr/86Sr of 0.705 ∓ 0.002. In addition to variation in initial 87Sr/86Sr between dykes, there is also variation between minerals within single dykes presumeably due to contamination during crystallization and/or deuteric alteration.

Hydrothermal alteration at the Middle Fork copper prospect, central Cascades, Washington

The Middle Fork copper prospect is in the central Cascade Range of Washington State, near the southeastern margin of the Miocene, composite Snoqualmie batholith. The principal zones of chalcopyrite mineralization and related wall rock alteration occur in medium-grained, equigranular, main-phase Snoqualmie granodiorite. The mineralized zone is characterized by en echelon, northeasterly trending shearing. Significant sulfide concentrations appear to occur in linear breccia zones commonly near intersections of northeasterly trending shears with steeply dipping, north to north-westerly striking cross faults.Geologic mapping, petrographic data, and whole rock chemical analyses suggest the following sequence of alteration and mineralization:(1) A northeasterly trending zone of unmineralized, quartz-deficient biotite-plagioclase breccia and peripheral quartz-stable, biotitic granodiorite breccia formed from main-phase Snoqualmie granodiorite during a period of presumed deuteric alteration. The removal of quartz from the quartz-deficient zones resulted in a volume loss of approximately 30 percent.(2) Subsequent introduction of solutions rich in potassium, iron, sulfur, copper, and minor molybdenum was restricted primarily to the quartz-deficient biotite-plagioclase breccias. The alteration took place at constant volume and resulted in an assemblage of hydrothermal orthoclase and secondary biotite associated with relict igneous plagioclase and primary biotite. Subequal amounts of disseminated chalcopyrite (with pyrrhotite blebs) and pyrite occur within this alteration zone. Retrogressive alteration of biotite to chlorite and plagioclase to sericite and carbonate minerals commonly is superimposed on the earlier assemblage.(3) A late-stage assemblage of sericite, pyrrhotite, quartz, carbonate, and chalcopyrite is especially well developed along northwesterly striking discontinuous shear zones. The sericite-pyrrhotite alteration occurred at constant volume and probably represents deposition from the same solutions responsible for the orthoclase alteration, but at decreased K (super +) /H (super +) ratios and lower temperatures.(4) Northerly to northwesterly striking post-mineralization faults cut the mineralized zone into discrete blocks.The Middle Fork deposit has hydrothermal alteration assemblages similar to southwestern United States porphyry copper deposits, but differs from them in that it (1) occurs in medium-grained quartz dioritic-granodioritic intrusive rocks of batholithic dimensions, (2) has linear, structurally controlled zones of alteration and mineralization, (3) has potassium silicate alteration assemblages remarkably deficient in quartz, (4) lacks a peripheral propylitic zone, (5) contains pyrrhotite as a major sulfide mineral, and (6) has virtually no supergene enrichment. Despite these differences, the Middle Fork deposit appears to belong within the broad spectrum of porphyry copper deposits although diverging widely from the unifying model of Lowell and Guilbert (1970).

Effect of sea water interaction on strontium isotope composition of deep-sea basalts

Analyses of rim-to-interior samples of fresh tholeiitic pillow basalts, deuterically altered holocrystalline basalts, and older, weathered tholeiitic basalts from the deep sea indicate that 87Sr/ 86Sr ratios of the older basalts are raised by low temperature interaction with strontium dissolved in sea water. 87Sr/ 86Sr correlates positively with H 2O in these basalts; however, there is little detectable modification of the strontium isotope composition in rocks with H 2O contents less than 1%. The isotope changes appear to be a function of relatively long-term, low-temperature weathering, rather than high-temperature or deuteric alteration. Strontium abundance and isotopic data for these rocks suggest that strontium content is only slightly modified by interaction with sea water, and it is a relatively insensitive indicator of marine alteration. Average Rb-Sr parameters for samples of apparently unaltered basalt are: Rb= 1.11ppm; Sr= 132ppm; 87Sr/ 86Sr= 0.70247.

A Tertiary Lamprophyre Dike Province in Southeastern Alaska

Mapping in southeastern Alaska along the British Columbia border has shown the presence of hundreds of aligned Tertiary Lamprophyre dikes. Most dikes occur in northeast-trending swarms that cut across northwest-trending structures in the bedrock. More than 60% of the dikes dip within 10° of vertical and strike between N35°E and N80°E. Major topographic features such as fjords and streams have the same trend; differential erosion along the dike swarms and parallel joint sets, not faulting, is responsible for this northeast topographic grain.The dikes show various degrees of deuteric alteration. In relatively unaltered rocks, phenocrysts are subcalcic augite and rarely plagioclase, altered olivine, or amphibole. Deuterically altered dikes have a texture of interlocking milky plagioclase and black amphibole needles.The dikes form an undifferentiated homogeneous suite of alkali olivine basalt composition with an average SiO2 content of 48.8% and a total alkali content of 5.3%. Their chemistry closely resembles Quaternary volcanic rocks in British Columbia and Alaska.Structural setting, extent, and chemistry of the dikes suggest a mantle source and rapid intrusion without time for differentiation.

Base Metal Distribution in Granitic Rocks; data from the Rocky Hill and Lights Creek Stocks, California

Significant systematic areal variation patterns in copper and zinc contents exist in rocks of the Rocky Hill (Tulare County) and Lights Creek (Plumas County) stocks, California. Significant areal variation patterns in copper contents of biotite and sulfide phases, total sulfide contents and other properties also exist in the Rocky Hill stock. These patterns are related, and an increase in the content of copper-bearing sulfides is commonly accompanied by a decrease in the copper content of the associated ferro-magnesian silicate phases. This relationship is usually independent of the total copper content of the rock and of the fractionation trends shown by other metals. The distribution patterns of total sulfides and the intensity of deuteric alteration, however, are related, with both increasing in content or intensity towards the core of the stock. Some copper may have been extracted from the silicate phases and fixed in sulfides during alteration. In spite of a low overall content of copper in the Rocky Hill stock, considerable concentration has occurred in a small area. The distribution of zinc in this stock does not appear to be significantly related to any of the above factors.Transfer or migration of copper within the rocks of the Rocky Hill stock is indicated by comparisons of the frequency distributions of copper and zinc and by other data. Consideration of these distribution patterns in relation to models of mineralization in granitic rocks points out that structural level and other aspects of the local geologic setting can have an important influence on the nature of such patterns. Interpretations in regard to metallogenesis and exploration should take this influence into account.Calculated activities of Cu, Zn, and annite molecule in biotite for estimated conditions of late crystallization at Rocky Hill appear to be in satisfactory agreement with observed analysis data. The total content of sulfur in the rock, including the vapor phase coexisting with it during crystallization, then becomes the critical factor in controlling sulfide-silicate partitioning of copper. Under the crystallization conditions estimated for Rocky Hill, a separate zinc sulfide phase is not stable.An analysis of the variation data for Cu and Zn in these stocks provides estimates of local scale or outcrop variation and establishes the significance of large scale (regional) variations occurring in both. It also provides a limited basis for an estimation of the type of distribution involved at the local or outcrop scale. As a result of fracture-controlled sulfide distribution in the Lights Creek stock, local scale variation is much larger than that at Rocky Hill and more dependent on sampling configuration; highly significant large scale variation in Cu and Zn is nonetheless present.

The origin of metal-bearing submarine hydrothermal solutions

Instrumental activation analyses for 16 major and trace elements in a suite of mid-Atlantic ridge basalts reveals that the slowly cooled interior portions of these submarine extrusions are depleted, relative to the quenched flow margins, in several elements that are enriched in pelagic sediments and manganese nodules (Mn, Fe, Co, the rare earth elements, and others). Many of these elements are excluded from the solid phases that crystallize from the melt, and thus are concentrated in residual liquids. Additional elements are mobilized during the deuteric alteration of early-formed olivine and the formation of immiscible sulfide liquids. It is suggested that these components of melt occupy accessible sites (e.g., intergranular boundaries) in the hot solid rock mass, and are mobilized by dissolution as chloride complexes in sea water introduced along contraction cracks that form during cooling and solidification. These solutions may be the metal-bearing ‘hydrothermal exhalations’ or ‘volcanic emanations’ that accompany submarine volcanism, which are often cited as a source of metals into the pelagic environment. Reasonable estimates of the amount of material involved suggest that a significant fraction of the mass of these elements that reside in pelagic sediments could have been supplied by this process. Recently described amorphous iron-manganese-silica material from the east Pacific rise may form by direct precipitation from these hydrothermal solutions following their introduction into sea water.

Geochemical study of trace elements in the alkalic rocks of Nemuro Peninsula, Hokkaido, Japan

Geochemical data for 13 samples of alkalic rocks from Nemuro Peninsula, Hokkaido, are presented and discussed. Analyses for trace elements, Ni, Co, Cr, Sc, Cu, Zn, Ga, Zr, Y, Pb, Ba, Sr and Li in a differentiated sheet ranging from picritic dolerite to syenite and in an undifferentiated dolerite sheet showing pillow structure, are given. It is shown that the Nemuro alkalic rocks have distinctly higher concentrations of Ba, Zn, Cu, Pb, Y and Li, high ratios of K/Sr and Ca/Sr, and lower concentrations of Sr, Ni, Co, Cr, Sc and Zr than do alkali basaltic rocks from other districts in the world. The behavior of the major and trace elements suggests that the Nemuro alkalic rocks were modified chemically by deuteric alteration during the magmatic stage. The parental potash-rich olivine basalt magma of these alkalic rocks was formed probably by partial melting of phlogopite-peridotite in the upper mantle.

Mineralogy and petrology of the diabasic rocks in a differentiated olivine diabase sill complex, Sierra Ancha, Arizona

The Precambrian Sierra Ancha sill complex, more than 700 feet thick, is a multiple intrusion with a central layer of feldspathic olivine-rich diabase, and upper and lower layers of olivine diabase derived from a high-alumina basalt magma. Minor rock types include albite diabase and albite-diabase pegmatite. Deuteric alteration was extensive. Principal primary minerals are plagioclase (An72 to An16), augite (Wo43En44Fs13 to Wo40En38Fs22), olivine (Fo74 to Fo54), orthopyroxene (En77 to En44), magnetite (Mgt66Usp34 to Mgt89Usp11), and ilmenite (Ilm86Hem14 to Ilm96Hem4). Ilmenite formed by reaction-exsolution from magnetitess is consistently different in compositon from primary ilmenite. Primary ilmenite became enriched in Mn and depleted in Mg as crystallization proceded. A systematic Fe-Mg partition between contacting olivine and orthopyroxene suggests that equilibrium prevailed on an extremely local scale during crystallization. Albite-diabase pegmatite contains a mineral assemblage including augite, ferrosalite (Wo49En28Fs23 to Wo49En14Fs37), albite (An2 to An0), and iron-rich chlorite. Altered diabase and albite diabase also have unusually calcium-rich pyroxenes. The calcium-rich pyroxenes, which occur in assemblages like those characterizing some spilites, are richer in calcium and lower in aluminum and titanium than basaltic augite.

Diabase Argillation at King Mountain, Kiowa County, Oklahoma

ABSTRACT The alteration of diabase dikes at King Mountain, Kiowa County, Oklahoma, illustrates the diversity of clay mineral alteration products which may result from very local variations in the micro-environment. Three diabase dikes exhibiting varying degrees of argillation are exposed in a one-hundred-foot-deep railroad cut. The diabase and its alteration products were examined by means of the petrographic microscope, X-ray diffraction aim electron microscopy. Chemical characterizations were obtained by means of chemical analysis by X-ray fluorescence differential thermal analysis and effluent gas analysis. Diabase from the center of the railroad cut contains 52% labradorite (An 59) and 31% chlorite. The chlorite is interpreted as a deuteric alteration product of the original pyroxene. As a weathering surface is approached at the east end of the railroad cut, the relatively fresh labradorite, chlorite and magnetite are replaced by goethite, interstratified illite-montmorillonite and kaolin. The oxidization and leaching has been accomplished by the downward percolation of groundwater along a steeply dipping dike. The same diabase dike contains interstratified chlorite-montmorillonite where relief is low and alkaline conditions persist. Tile clay mineralogy, clay mineral distribution and chemical data all support a weathering origin for the argillation. The clay mineral diversity has resulted from very local. relief-controlled variations in the chemical environment.

Geochemistry of precambrian diabase dikes from wyoming

Major and trace element concentrations in contact zones of 27 Precambrian diabase dikes from the Beartooth, Wind River, Bighorn, Owl Creek, and Laramie ranges in Wyoming indicate, in general, that the dikes are continental tholeiites. Relative to most young continental tholeiites, however, the Wyoming diabases are depleted in Sr relative to Rb, K, Ca, and usually, Ba and enriched in Rb relative to K and, sometimes, Ba; the Laramie dikes are also depleted in Ba relative to K. The Wyoming diabases (together with similar basaltic rocks from Antarctica and Tasmania and, perhaps, submarine tholeiites) define a fractionation trend in which Sr is rapidly depleted relative to Rb and K and enriched at a smaller rate relative to Ca (the Sr-depletion trend) as compared to the trend exhibited by most basalts (the normal-Sr trend). The existence of chilled contacts on the dikes and the lack of a systematic correlation between element distribution and secondary mineral content suggest that with respect to the trace elements investigated, secondary minerals in the dikes (primarily hornblende and chlorite) grew under essentially closed-system conditions (probably during deuteric alteration). Of the models considered plausible to explain the relative Sr-depletion and Rb-enrichment in the Wyoming diabases, that of extensive plagioclase crystallization from Ca-Al-rich, primary tholeiitic magmas are favored by existing data. If such a model is valid, the distribution of tholeiites exhibiting a Sr-depletion trend may offer an indirect method of mapping the distribution of Ca-Al-rich ultramafic source areas in the upper mantle. The Ba-depletion relative to K and Sr in the Laramie dikes is most readily explained in terms of a localized Ba-depleted upper-mantle source area.

Age of Rim Rock Dike Swarm, Trans-Pecos, Texas

Potassium-argon dates for basaltic rocks from the Rim Rock dike swarm of Trans-Pecos Texas range from 18 to 23 m.y. Because these dikes are closely associated with major normal faults, they indicate the age of Basin-Range faulting in the region. Two sets of dikes are distinguishable not only by strike and petrology, but also by age. The older, basaltic and trachy-basaltic set and the younger, more alkalic set have average ages of 22.3 and 19.2 m.y., respectively. Extensive deuteric alteration has not modified significantly the K-Ar dates of the rocks.