Leon E. Long

Stratigraphy and provenance of strata along the San Marcos fault, central Coahuila, Mexico

Stratigraphic and petrologic studies, supported by geologic mapping north of the San Marcos fault (central Coahuila), indicate that as much as 3,000 m of coarse Upper Jurassic and Lower Cretaceous detritus was derived from south of the fault. Two coarse Jurassic submarine conglomerate units (Las Palomas and Sierra El Granizo beds of the La Casita Formation) reflect separate episodes of displacement. Younger Jurassic eolianite and marine sandstone (Colorado and Tanque Cuatro Palmas beds of the La Casita Formation) record subsequent (Tithonian) fault inactivity. Neocomian rejuvenation of the fault produced alluvial deposits (San Marcos Formation); finer-grained, paralic or marine strata that are distal and suprajacent, and manifest the same faulting, are the La Mula Formation. Interruption of Neocomian displacement is indicated by a tongue of carbonate (Padilla and Barril Viejo Formations) that divides the San Marcos into upper and lower members. The principal source of this detritus was the wildflysch of the late Paleozoic, arcproximal Las Delicias basin and the plutonic rocks that cut it. Rb-Sr ages on clasts and from outcrops of granitic rocks are Triassic and probably reflect late-stage magmatism within the Las Delicias arc. Although this arc supplied most of the detritus to the Las Delicias basin, Devonian ages on some clasts reveal that an unknown source area lay somewhere nearby during the late part of the Paleozoic Era. The San Marcos fault may have served as part of the system of transform faults that connected sea-floor-spreading ridges of the Atlantic with those of the Pacific. If so, then the Jurassic units record periods of Jurassic activity and inactivity of that system. Neocomian displacment may have been caused by subsequent isostatic adjustment.

Rb-Sr dating of Paleozoic glauconite from the Llano region, central Texas

Abstract Glauconite from eight stratigraphic horizons (Cambrian to Pennsylvanian) in the Llano Uplift, central Texas and two Cretaceous glauconites were analyzed by the Rb-Sr method. Only two untreated samples provide ages in agreement with those anticipated from current best estimates of the geologic time scale. With one exception all the other apparent ages fall short of the estimated age of deposition by as much as 22%. Low ages, the pattern customarily observed, are attributed to postdepositional loss of radiogenic 87 Sr from expandable layers by weathering or during diagenesis. Detailed leaching experiments using a variety of reagents were performed to characterize the behavior of glauconite. The most promising treatment, which we recommend as standard procedure in all future studies, is with ammonium acetate which is able to purge the mineral of loosely-bound Rb and Sr while leaving tightly-bound components intact. After appropriate leach, three other Rb-Sr ages were brought into coincidence with their estimated ages of deposition. In contrast an Upper Cambrian glauconite was found to be extremely resistant to further alteration by chemical attack′, providing an age of 429 ± 17 M yr. Although 17% short of the age of deposition, this age is interpreted as the time of a real event: diagenetic recrystallization induced by burial. Comparison of data from four samples indicates that for Paleozoic glauconite, conditions exist in which the Rb-Sr system is less susceptible to mild disturbance than is the K-Ar system.

Rb-Sr and K-Ar geochronologic and isotopic studies, Llano Uplift, central Texas

Two major episodes are evident in the metamorphic and igneous Precambrian basement of the Llano Uplift, central Texas. Dynamothermal metamorphism was accompanied by minor basaltic and tonalitic syntectonic plutonism. This was followed by a second period of thermal overprinting accompanying emplacement of high-K2O, high-level major granite plutons. Extensive isotopic age work by Zartman, published in the mid-1960s, suggests that development of the basement complex, spanning an interval of 150 m.y. or more, began with deposition of Valley Spring Gneiss (the lowest unit) and terminated about 1,050 m.y. ago with final postmetamorphic cooling (indicated by retention ages of Ar and Sr in biotite). We have supplemented these data with more than 50 new K-Ar and Rb-Sr analyses.Two foliated plutons in the southeast are 1,167±12m.y. (2σ) old, with distinctly different initial 87Sr/86Sr ratios. Field relationships and isotopic data indicate that these plutons are the earliest yet known in the Uplift. Metamorphosed basalt dikes and gabbro bodies were emplaced immediately preceding and following the syntectonic plutons. Eleven of these rocks had extremely uniform initial 87Sr/ 86Sr=0.7029±0.0005. A Rb-Sr whole-rock isochron of the unfoliated Enchanted Rock pluton indicates an age of 1,048±34 m.y. with initial 87Sr/86Sr= 0.7048±0.0007. One of the northern unfoliated granites, the Lone Grove pluton, gives a whole-rock isochron age of 1,056±12 m.y., with initial 87Sr/86Sr = 0.7061±0.0003. All of the intrusive rocks have initial 87Sr/86Sr ratios consistent with a source in the mantle or lower crust, but not in ancient remobilized continental crust. Six K-Ar hornblende ages from metabasalts are 1,078±19 m.y. (1σ), in general agreement with K-Ar and Rb-Sr mineral ages elsewhere in the eastern Llano Uplift. A metasedimentary Valley Spring Gneiss sample from the western Uplift has a whole rock-muscovite Rb-Sr age of 1,129±9 m.y. Field and isotopic data are now sufficiently numerous to permit a moderately detailed reconstruction of the Precambrian history of the area.

Petrology and Rb-Sr isotope geochemistry of intrusions in the Diablo Plateau, northern Trans-Pecos magmatic province, Texas and New Mexico

Shallow intrusive bodies in a 65-km-long belt consist of quartz-bearing and feldspathoidal syenite, trachyte, and phonolite, all within a narrow range of composition. K-Ar ages (35 ± 2 m.y.) and modal, whole-rock chemical, Rb-Sr isotopic, and mineralogic data suggest that all the intrusive rocks were derived from one magma reservoir. Nine intrusive rock types are divisible into two groups. The older group, commonly forming sills and laccoliths, is fine- to coarse-grained and has little or no flow structure, low Rb, high Sr, and initial 87Sr/86Sr ratios of 0.703 to 0.709. The younger group, tending to form discordant sheets, is finely porphyritic and has strongly developed flow structure, high Rb, low Sr, and initial strontium ratios of 0.705 to 0.712. The younger group shows more extreme compositional variation, including the most silica-oversaturated and the most silica-undersaturated rocks. Each group contains both quartz-bearing and feldspathoidal rock types. Fractional removal of plagioclase and biotite from mugearitic liquid progressively enriched the remaining liquid in sodium while depleting it in calcium, magnesium, and titanium. Later differentiation proceeded mostly by removal of alkali feldspar.

Origin of granite at Cabo de Santo Agostinho, Northeast Brazil

A 4 km2 exposure of shallowly-emplaced leucogranite on the Atlantic coast at Cabo de Santo Agostinho, 30 km south of Recife, Brazil has been extensively studied chemically and isotopically. Twenty-three major-element analyses indicate that the Cabo granite ranges from peralkaline to peraluminous; Na2O+K2O is very high (7.4 to 10.4 wt.%), with CaO low (∼0.3%) and MgO vanishing (<0.06%). Microprobe analyses confirm the presence of arfvedsonite (biotite absent), and nearly total absence of plagioclase. The rocks are moderately to highly enriched in LREE (La 45 to 350 times chondritic), with extremely pronounced negative Eu anomalies (Eu/Eu*=0.02 to 0.07). Whole-rockδ18O is consistent at +8.5±0.3%.oSMOW A Rb-Sr isochron age of isotopically slightly disturbed samples is 104.8±1.8 Ma, with initial87Sr/86Sr=0.7084±0.0011. Sr is depleted (2–20 ppm) but Ba is 200–750 ppm.Crystallization path calculations and petrographic observations suggest that magma formed at a pressure close to 6 kbar but rose to a crustal level equivalent to roughly 1 kbar. Quartz, the liquidus phase at moderate H2O concentrations and pressures above 2 kbar, was resorbed during decompression as the magma moved upwards. Ultimately, quartz and alkali feldspar coprecipitated. Feldspar was not retained in the source rock nor removed early from the fractionating magma. Therefore the strong negative Eu anomaly and low Sr abundance are characteristics inherited from the source. A high H2O concentration necessary for a large degree of melting was lacking, hence the Cabo magma composition must reflect a small degree of partial melting of a rather quartz-rich rock such as a feldspathic arenite.In a pre-drift reconstruction of Gondwanaland, the Cabo granite fits on the southernmost and youngest end of the trend of the Niger-Nigerian igneous centers with which it has close affinity. The Cabo granite occupies the western end of the trace of the ancestral Ascension mantle plume which presumably served as the heat source.

History of recurrent activity along a major fault in northeastern Mexico

Mesozoic and Tertiary faulting within a 280-km-long, west-northwest–trending zone in Coahuila, Mexico, is inferred from stratigraphic and structural evidence. The fault forms the northern boundary of the Late Jurassic–Early Cretaceous Coahuila Island. Uplift on the south during the Jurassic and Early Cretaceous is indicated by a large detrital lithosome that formed on the northern side of the fault. Clast size within the Early Cretaceous part diminishes northward where the unit is divided into upper and lower parts by a limestone tongue. These two Early Cretaceous detrital units plus two Jurassic conglomerates at Valle San Marcos suggest episodic, pre-Tertiary movement. Deformation of 242 ± 2-m.y.-old granites at Valle San Marcos also reflects several periods of pre-Tertiary movement. Uplift on the north along the same fault zone during the Tertiary (Laramide) raised Permian and Jurassic rocks into juxtaposition with Cretaceous limestones. Extensive left-lateral movement is neither proven nor precluded. Nevertheless, because of the size, orientation, general location, and times of movement of the fault, we suggest that it may either be part of the megashear postulated by several authors for northern Mexico, or a splay of that megashear.

Rb-Sr Ages of Glauconite Recrystallization: Dating Times of Regional Emergence Above Sea Level

ABSTRACT Rb-Sr ages of glauconite pellets commonly fall short of the presumed age of deposition by 10-20%, according to current estimates of the geologic time scale. In part, the young ages are the result of preferential loss of exchangeable radiogenic Sr. Consequently, in pellets where tightly bound, nonexchangeable Rb and Sr have remained a closed chemical system since deposition, removal of the exchangeable cations with ammonium acetate (NH4OAc) can raise the apparent age to coincide with the expected depositional age. In other cases the age of pelletal glauconite remains younger than the depositional age after chemical treatment. Analyses of NH4OAc-treated, disaggregated clay-size fractions invariably yield young but rather consistent apparent ages. These are interpre ed as recording the time of diagenetic recrystallization of the glauconite crystallites which compose the pellets. Glauconite recrystallization appears to have taken place in the Llano Uplift of central Texas at 428, 350, 328, and 265-280 my ago. These ages do not coincide with times of elevated temperatures, deep burial, or tectonism. Instead, they correlate with times of regional emergence above sea level. Apparently a change in pore-water chemistry due to an influx of meteoric water promoted recrystallization, and this inference is supported by the oxygen isotope composition of the glauconite. Diagenesis of glauconite by this mechanism offers an opportunity to date sea-level changes by isotopic methods.

Rare-earth element geochemistry of the Meruoca and Mucambo plutons, Ceará, northeast Brazil

Abstract The Cambrian Meruoca and Mucambo plutons in Ceara, northeastern Brazil, are high-level crosscutting anorogenic intrusions with well-developed aureoles. The Meruoca pluton is predominantly alkalic to peralkalic granite, fayalite-bearing in part. Mucambo is comprised of porphyritic granite, quartz monzonite, quartz syenite and granite. A RbSr whole-rock isochron age for Mucambo is close to 550 m.y.; the Meruoca pluton may be of similar age, but its RbSr system has been disturbed by a prolonged, late hydrothermal event. Rare earth element (REE) abundances were determined on 5 whole-rock samples from each intrusion. All the REE fractionation patterns are similar, exhibiting smooth-trending enrichments of light REE and a prominent negative Eu anomaly. Concentrations of total REE are very much higher than for most granites, the enrichment of the lightest rare earths being as much as 1500 times that in chondrites. The field and laboratory data suggest that the Meruoca and Mucambo magmas originated by partial fusion of heterogeneous continental crust whose REE were already enriched and fractionated. Plagioclase was fractionated out of the melt prior to its final emplacement. Other granites in northeastern Brazil have similar REE patterns.

Petrologic and Rb-Sr Isotopic Study of the Chiquimula Pluton, Southeastern Guatemala

The Chiquimula pluton is one of a chain of plutons that extends across southern Guatemala and northern Honduras. Its exposed area of about 300 sq km in southeastern Guatemala is highly irregular. Exposed parts of the epizonal pluton are dominantly granodiorite; gabbro may be abundant, but it is exposed only in the deepest canyons. Diorite is predominant along the southern and western margins, and the rock is an intergrading mixture of granodiorite with subordinate adamellite and granite near the northern and eastern margins. A whole-rock Rb-Sr isochron diagram shows data points falling in two distinct groups. Granite and adamellite form a scattering of points indicating relatively high Rb/Sr (3.3 to 4.6) and high Sr 87 /Sr 86 (0.72 to 0.75). Points from the volumetrically more important gabbro and grand diorite, indicative of low Rb/Sr (1.2 or less) and Sr 87 /Sr 86 (0.706 to 0.708), form a well-defined isochron corresponding to an age of 50 ± 5 m.y. and initial Sr 87 /Sr 86 = 0.7060 ± 0.0002. K-Ar and Rb-Sr biotite ages from granite of 84 m.y. and 95 m.y., respectively, are younger than the whole-rock Rb-Sr age (215 m.y.) of the same specimen, but older than the age of emplacement of the pluton as suggested by the whole-rock isochron from granodiorite and gabbro. Field relations, petrographic data, and isotopic data are consistent with the interpretation that the Chiquimula pluton is just beginning to be unroofed. The compositional variation probably resulted from a combination of assimilation and differentiation of a granodioritic magma. The more granitic parts may be due to partially assimilated more siliceous exotic blocks of basement rock.

Isotope dilution analysis of common and radiogenic strontium using84Sr-enriched spike

Abstract In determining the quantities of common and radiogenic strontium from a single isotope dilution analysis, a fractionation correction can be applied to observed isotope ratios if the spike is enriched in 84 Sr. However, for certain combinations of the isotopic composition of the spike and the spike to sample ratio, the analysis is subject to infinite error magnification. A consideration of the magnification of error indicates that nearly pure 84 Sr spike is optimum.