Local Basement Rocks Research Articles

Locating the Yangtze Block in Nuna: Constraints from age and isotopic data from Paleoproterozoic sedimentary rocks in the Phan Si Pan Zone, northwest Vietnam

New U-Pb ages and Hf isotopic data for detrital zircons from the late Paleoproterozoic sedimentary rocks (Suoi Chieng Formation) in the Phan Si Pan Zone of northwest Vietnam constrain their depositional ages, provenance, and tectonic setting. The weighted average age of the youngest coherent zircon population is 1769 ± 31 Ma (MSWD = 1.3, n = 8), which provides a maximum depositional age for the Suoi Chieng Formation. The detrital zircons obtained from six Paleoproterozoic samples exhibit various ages, showing multiple magmatic and metamorphic events. The detrital zircon age pattern shows that magmatic activities in the source areas occurred at 2650 Ma, 2350–2150 Ma, and 2050 Ma, as well as minor activities at 2850 Ma, 1850 Ma, and 1780 Ma. Additionally, a prominent metamorphic age peak is observed around ca. 1850 Ma, while a few metamorphic zircons exhibit ages ranging from 2600 to 2500 Ma and 2100 Ma. This pattern indicates that the primary source of these zircons is the local basement rocks of the Phan Si Pan Zone and SW Yangtze. However, contributions from other adjoining cratonic blocks are also possible and these likely included the Rae craton of North Laurentia. Our new data verifies and expands earlier results, showing that the SW Yangtze and north Yangtze have distinct Archean-Paleoproterozoic crustal evolutionary histories prior to their assembly in the late Paleoproterozoic. Regional correlations based on detrital zircon spectra, stratigraphic and tectono-thermal histories show strong similarities between the SW Yangtze and the Rae craton of north Laurentia throughout the Paleoproterozoic, suggesting the proximity of the two blocks before or during the formation of the Nuna supercontinent. We further suggest that the north Yangtze adjoined southern Siberia in the Archean to late Paleoproterozoic as supported by their identical tectono-thermal histories and detrital zircon age spectra. In combination with published data, we propose that the Yangtze Block was located between north Laurentia and south Siberia during the Nuna assembly.

A High-δ18O Mantle Source for the 2.06 Ga Phalaborwa Igneous Complex, South Africa?

ABSTRACTThe 2060 ± 2 Ma Phalaborwa Complex is a pipe-like, ultramafic to carbonatite intrusion formed from multiple magma pulses. The complex is made up of a main pipe consisting of clinopyroxenites, ultramafic pegmatoids, carbonatites and foskorite (olivine-apatite-magnetite-calcite assemblage), surrounded by many smaller syenite plugs. The range in mineral δ18O values for all rock types and minerals analysed in the Phalaborwa Complex is 2.24 to 18.3‰. However, 24 analyses of the most abundant and robust mineral, diopside, all have δ18O values between 6.5 and 8.1‰. The δ18O values of baddeleyite, olivine, diopside, magnetite, apatite and aegirine are thought to be magmatic. Most mineral pairs have differences in δ18O values that are consistent with magmatic equilibrium at high temperatures down to closure temperature. Alkali feldspar and phlogopite have more variable δ18O values, and both minerals may have undergone subsolidus O exchange. The δD values for petrographically fresh Phalaborwa Complex phlogopite range from −79 to −48‰ with a mean of −64 ± 9‰ (1σ, n = 19). The phlogopite δD values are consistent with subduction-related magmatic water. Despite petrographic evidence for fluid–rock interaction in the carbonatite–foskorite rocks, the carbonatite δ13C and δ18O range (δ18O, 8.13 to 12.00‰; δ13C, −3.19 to −5.60‰) overlaps with the unaltered, primary igneous carbonatite field.Magma δ18O values estimated from silicate and oxide minerals are mostly higher than normal mantle magmas (pyroxenites, ~7.6‰; foskorite, 7.2‰). The δ18O value of syenite magma estimated from aegirine is 7.8 ± 0.9‰ (1σ, n = 8), in equilibrium with whole-rock syenite δ18O values (8.7 ± 0.4‰, 1σ, n = 5). Local basement rocks have average bulk δ18O values of 8.6‰, and realistic proportions of assimilation by a mantle-derived magma (δ18O, 5.7‰) could not have produced the δ18O values in the pyroxenites or foskorites. Instead, it is proposed that the high-δ18O values of Phalaborwa Complex magmas reflect that of the mantle source. High δ18O values are also a feature of the Rustenburg Layered Suite of the Bushveld Complex (2060 to 2055 Ma), which may have had a similar high-δ18O mantle source. The higher δ18O values of the satellite syenites are consistent with an origin by partial melting of metasomatised country rock.

Variation of Cu, Fe, S and Pb isotopes in sulfides from hydrothermal mineralization from the Yenice region in Çanakkale, Biga Peninsula, NW Turkey

We report the iron (n = 13), copper (n = 13), sulfur (n = 25) and lead (n = 31) isotope compositions of mineralized samples from porphyry, skarn and epithermal mineralization in the Yenice region that lies in the east of the Biga Peninsula, Turkey. The vast majority of isotopic data were analyzed from the world-class Arapuçandere Pb-Zn-Cu (Ag-Au) deposit that is one of the best example of intermediate sulfidation epithermal deposits within the Serbo-Macedonian-Rhodope metallogenic belt in the northeastern of the Aegean. The δ56Fe values range from −1.06 to + 0.46‰ and reflect the complex interplay between redox, Rayleigh and kinetic effects. The δ65Cu values also define a narrow ranges of −0.07 to +0.77‰ that is typical of primary minerals in magmatic-hydrothermal ore deposits worldwide. The Cu isotope values of chalcopyrite from the Arapuçandere deposit tend to progressively increase through the surface, which creates a vector from the proximal to distal portions of magmatic-hydrothermal environment, possibly with local supergene processes within deposit. The sulfides from the Yenice region have a relatively narrow range of δ34S values that range between −4.0 and 3.9‰ and mostly cluster around 0‰, indicating sulfur is mainly derived from a magmatic source at the time of mineralization. All the Pb isotope data that includes results from ore minerals fall within the fields defined by Oligocene to Miocene Aegean-West Anatolian magmatic rocks. This observation supports the hypothesis that the metals and sulfur contained within all the various styles of mineralization considered here were derived entirely from these granitoids, with no significant contamination from the local basement rocks. This observation also constrains the processes controlling the Fe and Cu isotope compositions to those operating within the porphyry, skarn, and epithermal hydrothermal systems, rather than contamination by interaction with the basement rocks.

Sediment grain size does matter: implications of spatiotemporal variations in detrital zircon provenance for early Paleozoic peri-Gondwana reconstructions

A comprehensive understanding of the relationship between the Sino-Korean Craton (SKC) and the Gondwana supercontinent is crucial for accurate reconstruction of the tectonic evolution and early Paleozoic paleogeography of East Asia. To explore the link between the SKC and peri-Gondwana, we provide new detrital zircon U–Pb age data from litho- and bio-stratigraphically constrained lower Cambrian to Lower Ordovician sandstone deposits from the eastern margin of the SKC (Taebaeksan Basin). Results indicate that the two distinct age spectra of detrital zircon resulted from provenance change combined with a strong function of sediment grain size within host siliciclastic rocks of the Taebaek Group. The age spectra from coarse-grained sandstones display Paleoproterozoic (1.9 Ga) and Neoarchean (~ 2.5 Ga) peaks, indicating that the sediments were supplied from the basement rocks of the SKC. Conversely, age spectra from fine-grained siliciclastics contain late Mesoproterozoic (~ 1.0 Ga) and Neoproterozoic (~ 0.6 Ga) peaks, with or without a Paleoproterozoic signal, consistent with a Gondwanan sediment origin. These different age populations have been documented in multiple Cambrian and Ordovician sequences of the SKC and are consistently well correlated with sediment grain size. Coarse-grained sediments, sourced primarily from local basement rocks, were deposited in coastal, nearshore, and shelf environments, whereas fine-grained sediments were derived from more distal sources (i.e., the Gondwana mainland and/or the Gyeonggi Marginal Belt) and deposited in inner to outer shelf settings. Therefore, the repeated occurrence of sediment with two distinct provenances in the Cambrian–Ordovician siliciclastics of the SKC is likely a result of provenance shift integrated with changing depositional environments in an epeiric platform. This yielded variations in sediment grain size and source rock provenance, with and/or without a tectonic activity. Consequently, our results indicate that the SKC was adjacent to Gondwana during the early Paleozoic.

Sandstone provenance and sediment dispersal in a complex tectonic setting: Taranaki Basin, New Zealand

Mineralogical, geochemical and detrital zircon data from key reservoir intervals in Taranaki Basin are integrated with paleogeography and basement maps to provide a synthesis of changes in sediment provenance and sandstone composition through time. Results show that sandstone composition and quality in Taranaki is strongly related to sediment provenance and basin history. Late Cretaceous syn-rift deposits display clear evidence for first-cycle erosion from local basement rocks, dominated by granites of the Median Batholith. ANE-SW trending shoreline belt developed through the Paleocene, which was associated with a gradual increase in sandstone maturity that peaked during the Eocene passive margin phase. Sediment provenance continued to be primarily from granitoids with transport directions from the S/SW to N/NE. Progressive quartz enrichment is interpreted to be a result of cleaning through longshore drift/longer transport distances, and enhanced chemical weathering due to warmer paleoclimates. We also invoke a component of eroded and recycled Cretaceous-Paleocene sediment linked to an episode of uplift in the south.A transition period ensued from the Late Eocene to Early Miocene associated with a change in basin tectonics from passive margin to active margin. Compositional and detrital zircon U-Pb data indicate that rocks to the east of Taranaki Basin became emergent, with progressive erosion of Cretaceous-Eocene cover rocks and underlying basement terranes. Minor, southward-propagating uplift of the eastern basin margin was initiated in the latest Eocene, and by the Late Oligocene, provenance and sedimentation patterns had clearly changed with the earliest Miocene strata sourced from eastern metasedimentary basement rocks. The Eastern Province continued to be a major sediment source through the active margin phase, but mixed with the re-appearance of granite (?reworked) sources and Miocene volcaniclastic detritus. These changes in sediment provenance through time are displayed on a set of eight sediment dispersal maps, which can be used to help predict composition, and hence reservoir quality, in undrilled parts of the basin.

Polyphase vein mineralization in the Fennoscandian Shield at Åkerlandet, Järvsand, and Laisvall along the erosional front of the Caledonian orogen, Sweden

The Akerlandet, Jarvsand, and Laisvall deposits in Sweden are calcite-fluorite-sulfide vein deposits and occurrences located close to the current erosional front of the Caledonian orogen and hosted by crystalline basement rocks in the Fennoscandian Shield. At Laisvall, basement-hosted veinlets occur beneath Ediacaran to Cambrian sandstones that host a strata-bound Pb-Zn deposit. The mineralized fractures at Akerlandet and Jarvsand occur along fault systems oriented N–S to NNW–SSE. Veins or veinlets strike NNW–SSE and NW–SE at Akerlandet, NNE–SSW at Jarvsand, and NNW–SSE and NNE–SSW to NE–SW at Laisvall. At Akerlandet and Jarvsand, fractures acted as conduits for hydrothermal fluids of variable composition and formed during separate tectonic events. At Akerlandet, the fault zone with NNW–SSE strike shows kinematic indicators consistent with ~NE–SW bulk horizontal extension. At Jarvsand, the calcite-fluorite-galena veins formed along R-Riedel shears related to the host N–S to NNW–SSE fault system. The kinematic indicators are consistent with ~NW–SE bulk horizontal extension, similar to the extensional deformation during the later part of the Caledonian orogeny (Silurian to Devonian). At Akerlandet, adularia-quartz deposition was followed by sphalerite ± galena and finally by precipitation of fluorite and calcite. 40Ar-39Ar thermochronology of a single adularia sample did not yield a well-defined plateau age but the gas released at higher temperatures suggests an early Tonian (980 to 950 Ma) crystallization age, i.e., during the later part of the Sveconorwegian orogeny, although the data do not exclude other less likely interpretations. Previous fluid inclusion microthermometry and geochronological studies and new petrographic and geochemical results suggest that sphalerite ± galena mineralization formed from saline, relatively oxidizing, moderate-temperature, and slightly acidic hydrothermal fluids, either during the Ediacaran or the Middle Ordovician. Metals and H2S were derived from local basement rocks. Based on petrographic evidence, rare earth element composition, and S, C, and O isotope data, fluorite and calcite precipitated under near neutral and relatively reducing conditions. Occurrence of solid bitumen in veins at Akerlandet and C and O isotope data of calcite at Akerlandet and in the Laisvall basement veinlets suggest that the precipitation of calcite and fluorite was triggered by interaction of hot and evolved hydrothermal fluids (87Sr/86Sr = 0.718–0.732) with organic matter. Structural, petrographic, and geochemical data at Laisvall suggest that the basement structures hosting calcite-fluorite ± pyrite veinlets were utilized in the Middle Ordovician as the plumbing system for the oxidizing, slightly acidic, metal-bearing brines that caused the economic Pb-Zn mineralization in the overlying sandstones.

Detrital zircon U-Pb geochronology and Hf isotope geochemistry of Paleozoic and Triassic passive margin strata of western North America

U-Pb geochronologic and Hf isotopic analyses have been conducted on detrital zircons extracted from 36 samples of Neoproterozoic through Triassic passive margin strata from western North America. The data serve as an improved reference for comparison with inboard strata that accumulated on the North American craton and outboard strata belonging to potentially displaced Cordilleran terranes. As expected, this reference documents significant variations in ages and Hf isotope compositions both north-south and also through time. The data also provide insights into the provenance of Cordilleran passive margin strata. During Neoproterozoic, Cambrian, and Early–Middle Devonian time, most grains were shed from relatively local basement rocks and from Mesoproterozoic clastic strata containing 1.2–1.0 Ga grains that originated in the Grenville orogen. This pattern was interrupted during Ordovician time, when much of the Cordilleran margin was blanketed by detritus shed from the northern Canadian Shield. Beginning in Late Devonian time, and continuing through late Paleozoic and Triassic time, most regions were dominated by locally derived detritus (largely recycled from underlying strata), but also received 0.7–0.4 Ga grains that were shed from the Franklinian, Caledonian, Appalachian, and Ouachita-Marathon orogens. This pattern is complicated in southern transects as a result of mid-Paleozoic emplacement of off-shelf assemblages onto the continental margin (e.g., Antler orogeny) and construction of Permo-Triassic magmatic arcs along the margin. Our data also provide a robust record of the crustal evolution of western North America, with significant production of juvenile crust during late Archean (3.0–2.5 Ga) and Paleoproterozoic (1.78–1.6 Ga) time and phases of mainly crustal reworking at 2.0–1.78, 1.5–1.3, 1.2–1.0, and 0.6–0.2 Ga. This history is somewhat different from that of other continents, with western Laurentia comprising a greater overall proportion of juvenile crust, punctuated by greater degrees of crustal reworking between 2.2 and 1.78 Ga and 0.3–0.2 Ga.

Sedimentology and palaeoecology of lonestone-bearing mixed clastic rocks and cold-water carbonates of the Lower Permian Basal Beds at Fossil Cliffs, Maria Island, Tasmania (Australia): Insight into the initial decline of the late Palaeozoic ice age

Abstract The middle Sakmarian Basal Beds on Maria Island were deposited during the initial decline of the Late Palaeozoic Ice Age following Late Pennsylvanian–Early Sakmarian maximum glaciation. At that time, Tasmania was located within the South Polar Circle between an apparent ice-free pole (Antarctica) and the mid- to high-latitude Sydney-Bowen-Gunnedah glacigenic basins in eastern Australia. The dropstone-bearing Basal Beds consist of: interstratified siltstone, conglomerate and cold-water limestones of the Lower Erratic Zone; siltstone and Eurydesma- rich cold-water carbonates of the Darlington Limestone; and siltstone and conglomerate of the Upper Erratic Zone. Interstratification of the coarse-clastic strata, siltstones and limestones within these units were previously attributed to glacial/non-glacial cycles. However, the interfingering of beds within each of these units and the occurrence of large, fossil bryozoans crossing and abutting lithological boundaries indicate that cyclicity was of shorter duration than that of Milankovitch-driven cycles. Within these intertidal and subtidal deposits, the occurrence of rounded dropstones derived from local basement rocks exposed along a rocky coastline and an absence of glacial indicators other than dropstones, along with other evidence, suggest that ice rafting was by sea ice rather than by icebergs. Study results confirm the spatial restriction of Middle Sakmarian to earliest Wuchiapingian glaciation.

The Witputs diamictite in southern Namibia and associated rocks: constraints for a global glaciation?

The Witputs section of the Gariep Belt (S Namibia) comprises a sequence of clastic and chemical sediments, which have been interpreted as representative of a Late Neoproterozoic global or near-global ice age event, and recent biostratigraphic work in the upper rocks of the Witputs suggest a Late Ediacaran age. To further characterise this sequence and provide additional age constraints, a detailed sedimentological and detrital zircon study has been carried out. The petrographic, sedimentological and geochemical characteristics of the Witputs diamictite determined in this study are homogenous and indicative of debris flow or palaeo-valley infill sediments, deposited in an oxic environment with no glaciogenic evidence. This homogeneity is also reflected in the detrital zircon age spectra with most ages falling between 1.0 and 1.3 Ga, representing the local geology, with the youngest grain at 1030.2 ± 10.9 (2σ) (n = 92 <10% discordance), despite the fact that mid and Late Neoproterozoic volcanic activity is known in the local region. The overlying carbonate rocks, often considered to be ‘cap carbonates’, show high Mn (up to 60% MnO), with base metal precipitation (Zn, V, Co), and are recrystallised. Their δ13CVPDB isotope ratios are homogeneous at around-3. Major and trace element ratios reach values which indicate that C–O isotopes may be disturbed and might not reflect primary global seawater composition, thus questioning their use for global correlation and comparison with composite chemostratigraphic curves. The contact to the overlying Late Ediacaran Sanddrif Member is not exposed, and the rocks dip in a different direction than the underlying carbonate rocks. The c. 40-m-thick section is characterised by rapid lithology changes including shales, calcareous sandstones and wackes, fine-grained conglomerates and rare clean quartz-rich sandstones, all of which have strikingly similar detrital zircon populations, and the youngest zircon is dated at 1082.8 ± 10 Ma (2σ errors, from 72 grains with <10% discordance). Acritarchs earlier found in the Sanddrif Member, however, indicate a post-570 Ma depositional age. If the diamictites are glacio-marine deposits, then an interesting conclusion is that the clastic sediments can display a very immature geochemical signature, indicating a localised provenance, with derivation purely from the local basement rocks, which is also reflected in the detrital zircon populations. However, we would hesitate to assign a glacial origin to the deposits as no glacial indicators, other than a diamictitic texture, were observed. Clearly, far more work on the detailed mapping and sedimentology of the Neoproterozoic Gariep Belt deposits is required, particularly as many are currently used for global correlation. Age constraints derived from extensive detrital zircon work can only constrain the deposits as being post 1.03 Ma with the detritus being purely locally derived.

Basic volcanism contemporaneous with the Sturtian glacial episode in NE Scotland

ABSTRACTThe Dalradian Supergroup contains three distinct glacigenic units, formerly termed ‘Boulder Beds’, which are correlated with widespread Neoproterozoic glaciations. The oldest and thickest unit, the Port Askaig Formation, marks the Appin–Argyll group boundary of the Dalradian Supergroup and has been correlated with the Middle Cryogenian (Sturtian) glaciation. The Auchnahyle Formation, a diamictite-bearing sequence near Tomintoul in NE Scotland, exhibits strong lithological similarities to the Port Askaig Formation. Both these glacigenic ‘Boulder Bed’ units contain abundant dolomite clasts in their lower parts and more granitic material at higher levels. Both metadiamictite units are overlain by thick shallow-marine quartzite units. C isotope data from Appin Group carbonate strata below the Auchnahyle Formation support this correlation. U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) detrital zircon data from the Auchnahyle Formation metadiamictite differ slightly from the Port Askaig Formation, but are similar to detrital zircon spectra obtained from the Macduff Formation, a diamictite unit in the younger Southern Highland Group of the Dalradian Supergroup; both apparently reflect derivation from local basement rocks. No detritus younger than 0·9 Ga is observed, so the data do not constrain significantly the depositional age of the glacial strata. A thin tholeiitic pillow basalt unit in the lower part of the Auchnahyle Formation is geochemically distinct from pre-tectonic metadolerite sills and from basic metavolcanic rocks up-section. A Sturtian (c. 720–700 Ma) age for the Auchnahyle Formation metadiamictite would imply that this basaltic volcanism represents the oldest recorded volcanic activity in the Dalradian Supergroup and is inferred to represent an early, local phase of proto-Iapetan rifting within the Rodinian supercontinent.

Chemical and boron-isotope variations in tourmalines from an S-type granite and its source rocks: the Erongo granite and tourmalinites in the Damara Belt, Namibia

Tourmaline is widespread in metapelites and pegmatites from the Neoproterozoic Damara Belt, which form the basement and potential source rocks of the Cretaceous Erongo granite. This study traces the B-isotope variations in tourmalines from the basement, from the Erongo granite and from its hydrothermal stage. Tourmalines from the basement are alkali-deficient schorl-dravites, with B-isotope ratios typical for continental crust (δ11B average −8.4‰ ± 1.4, n = 11; one sample at −13‰, n = 2). Virtually all tourmaline in the Erongo granite occurs in distinctive tourmaline-quartz orbicules. This “main-stage” tourmaline is alkali-deficient schorl (20–30% X-site vacancy, Fe/(Fe + Mg) 0.8–1), with uniform B-isotope compositions (δ11B −8.7‰ ± 1.5, n = 49) that are indistinguishable from the basement average, suggesting that boron was derived from anatexis of the local basement rocks with no significant shift in isotopic composition. Secondary, hydrothermal tourmaline in the granite has a bimodal B-isotope distribution with one peak at about −9‰, like the main-stage tourmaline, and a second at −2‰. We propose that the tourmaline-rich orbicules formed late in the crystallization history from an immiscible Na–B–Fe-rich hydrous melt. The massive precipitation of orbicular tourmaline nearly exhausted the melt in boron and the shift of δ11B to −2‰ in secondary tourmaline can be explained by Rayleigh fractionation after about 90% B-depletion in the residual fluid.

Zircon and whole‐rock Nd‐Pb isotopic provenance of Middle and Upper Ordovician siliciclastic rocks, Argentine Precordillera

AbstractGraptolite‐bearing Middle and Upper Ordovician siliciclastic facies of the Argentine Precordillera fold‐thrust belt record the disintegration of a long‐lived Cambro‐Mid Ordovician carbonate platform into a series of tectonically partitioned basins. A combination of stratigraphic, petrographic, U‐Pb detrital zircon, and Nd‐Pb whole‐rock isotopic data provide evidence for a variety of clastic sediment sources. Four Upper Ordovician quartzo‐lithic sandstones collected in the eastern and central Precordillera yield complex U‐Pb zircon age spectra dominated by 1·05–1·10 Ga zircons, secondary populations of 1·22, 1·30, and 1·46 Ga, rare 2·2 and 1·8 Ga zircons, and a minor population (&lt;2%) of concordant zircons in the 600–700 Ma range. Archaean‐age grains comprise &lt;1% of all zircons analysed from these rocks. In contrast, a feldspathic arenite from the Middle Ordovician Estancia San Isidro Formation of the central Precordillera has two well‐defined peaks at 1·41 and 1·43 Ga, with no grains in the 600–1200 Ma range and none older than 1·70 Ga. The zircon age spectrum in this unit is similar to that of a Middle Cambrian quartz arenite from the La Laja Formation, suggesting that local basement rocks were a regional source of ca 1·4 Ga detrital zircons in the Precordillera Terrane from the Cambrian onwards. The lack of grains younger than 600 Ma in Upper Ordovician units reinforces petrographic data indicating that Ordovician volcanic arc sources did not supply significant material directly to these sedimentary basins. Nd isotopic data (n = 32) for Middle and Upper Ordovician graptolitic shales from six localities define a poorly mixed signal [ɛNd(450 Ma) = −9·6 to −4·5] that becomes more regionally homogenized in Upper Ordovician rocks (−6·2 ± 1·0; TDM = 1·51 ± 0·15 Ga; n = 17), a trend reinforced by the U‐Pb detrital zircon data. It is concluded that proximal, recycled orogenic sources dominated the siliciclastic sediment supply for these basins, consistent with rapid unroofing of the Precordillera Terrane platform succession and basement starting in Mid Ordovician time. Common Pb data for Middle and Upper Ordovician shales from the western and eastern Precordillera (n = 15) provide evidence for a minor (&lt;30%) component that was likely derived from a high‐μ (U/Pb) terrane.

Eruption and magma crystallization ages of Las Tres Vírgenes (Baja California) constrained by combined 230Th/ 238U and (U–Th)/He dating of zircon

Las Tres Vírgenes volcano is a calc-alkaline composite cone located near the main Gulf of California escarpment on the E coast of the Baja California peninsula. High-sensitivity ion microprobe U-series ( 230Th/ 238U) ages for zircon from La Vírgen tephra average 121 − 10 + 12 ka (1σ; MSWD = 2.7), with discrete age peaks at ∼ 100 and 160 ka. The noble gas mass spectrometric (U–Th)/He zircon age, corrected for disequilibrium and pre-eruptive storage, is 36 ± 3 ka. This result for the eruption age of La Vírgen tephra is significantly older than previously postulated historic or Holocene ages that were based on an 18th century map reference and 14C dating of accidental charcoal, respectively. The new (U–Th)/He zircon age is consistent with a > 26 ± 4 ka age derived from cosmogenic He exposure dating of an overlying basaltic lava flow [Hausback, B.P. and Abrams, M.J., 1996. Plinian eruption of La Virgen Tephra, Volcán Las Tres Virgenes, Baja California Sur, Mexico. Eos, Transactions, American Geophysical Union, 77(46, Suppl.): 813–814.]. U–Pb zircon analysis of ignimbrites erupted from the adjacent Early Pleistocene La Reforma and El Aguajito calderas yielded ages of 1.38 ± 0.03 Ma ( n = 12; MSWD = 1.0) and 1.17 ± 0.07 Ma ( n = 23; MSWD = 1.3), respectively. No evidence for these ages is found among La Vírgen zircons, whereas pre-Quaternary zircon xenocrysts are common. The La Vírgen magma, therefore, evolved unrelated to Early Pleistocene magmatism in adjacent calderas, but assimilated local basement rocks. A gap between average Th–U and (U–Th)/He zircon ages suggests that zircon crystallization was discontinuous in the La Vírgen magma chamber. In addition, partial resorption of zircon suggests episodic thermal rejuvenation, most likely by basaltic recharge. Based on the zircon record, the > 100 ka lifetime of the thermal anomaly that sustained repeated intrusive pulses significantly exceeds the age of the last eruption. This strengthens the view that Tres Vírgenes has a potential for future eruptions.

Precipitation of lead–zinc ores in the Mississippi Valley‐type deposit at Trèves, Cévennes region of southern France

AbstractThe Trèves zinc–lead deposit is one of several Mississippi Valley‐type (MVT) deposits in the Cévennes region of southern France. Fluid inclusion studies show that the ore was deposited at temperatures between approximately 80 and 150°C from a brine that derived its salinity mainly from the evaporation of seawater past halite saturation. Lead isotope studies suggest that the metals were extracted from local basement rocks. Sulfur isotope data and studies of organic matter indicate that the reduced sulfur in the ores was derived from the reduction of Mesozoic marine sulfate by thermochemical sulfate reduction or bacterially mediated processes at a different time or place from ore deposition. The large range of δ34S values determined for the minerals in the deposit (12.2–19.2‰ for barite, 3.8–13.8‰ for sphalerite and galena, and 8.7 to −21.2‰ for pyrite), are best explained by the mixing of fluids containing different sources of sulfur. Geochemical reaction path calculations, based on quantitative fluid inclusion data and constrained by field observations, were used to evaluate possible precipitation mechanisms. The most important precipitation mechanism was probably the mixing of fluids containing different metal and reduced sulfur contents. Cooling, dilution, and changes in pH of the ore fluid probably played a minor role in the precipitation of ores. The optimum results that produced the most metal sulfide deposition with the least amount of fluid was the mixing of a fluid containing low amounts of reduced sulfur with a sulfur‐rich, metal poor fluid. In this scenario, large amounts of sphalerite and galena are precipitated, together with smaller quantities of pyrite precipitated and dolomite dissolved. The relative amounts of metal precipitated and dolomite dissolved in this scenario agree with field observations that show only minor dolomite dissolution during ore deposition. The modeling results demonstrate the important control of the reduced sulfur concentration on the Zn and Pb transport capacity of the ore fluid and the volumes of fluid required to form the deposit. The studies of the Trèves ores provide insights into the ore‐forming processes of a typical MVT deposit in the Cévennes region. However, the extent to which these processes can be extrapolated to other MVT deposits in the Cévennes region is problematic. Nevertheless, the evidence for the extensive migration of fluids in the basement and sedimentary cover rocks in the Cévennes region suggests that the ore forming processes for the Trèves deposit must be considered equally viable possibilities for the numerous fault‐controlled and mineralogically similar MVT deposits in the Cévennes region.

Evolution of a complex isolated dome system, Cerro Pizarro, central M�xico

Cerro Pizarro is an isolated rhyolitic dome in the intermontane Serdan-Oriental basin, located in the eastern Trans-Mexican Volcanic Belt. Cerro Pizarro erupted ~1.1 km3 of magma at about 220 ka. Activity of Cerro Pizarro started with vent-clearing explosions at some depth; the resultant deposits contain clasts of local basement rocks, including Cretaceous limestone, ~0.46-Ma welded tuff, and basaltic lava. Subsequent explosive eruptions during earliest dome growth produced an alternating sequence of surge and fallout layers from an inferred small dome. As the dome grew both vertically and laterally, it developed an external glassy carapace due to rapid chilling. Instability of the dome during emplacement caused the partial gravitational collapse of its flanks producing various block-and-ash-flow deposits. After a brief period of repose, re-injection of magma caused formation of a cryptodome with pronounced deformation of the vitrophyric dome and the underlying units to orientations as steep as near vertical. This stage began apparently as a gas-poor eruption and no explosive phases accompanied the emplacement of the cryptodome. Soon after emplacement of the cryptodome, however, the western flank of the edifice catastrophically collapsed, causing a debris avalanche. A hiatus in eruptive activity was marked by erosion of the cone and emplacement of ignimbrite derived from a caldera to the north of Cerro Pizarro. The final growth of the dome growth produced its present shape; this growth was accompanied by multiple eruptions producing surge and fallout deposits that mantle the topography around Cerro Pizarro. The evolution of the Cerro Pizarro dome holds aspects in common with classic dome models and with larger stratovolcano systems. We suggest that models that predict a simple evolution for domes fail to account for possibilities in evolutionary paths. Specifically, the formation of a cryptodome in the early stages of dome formation may be far more common than generally recognized. Likewise, sector collapse of a dome, although apparently rare, is a potential hazard that must be recognized and for which planning must be done.

Mingling of Immiscible Dolomite Carbonatite and Trachyte in Tuffs from the Massif Central, France

A pyroclastic tuff from the Velay volcanic province in the French Massif Central contains blocks up to 30cm long of local basement rocks, lava clasts, coarse-grained cumulates and pyroclastic fragments, with more or less diffuse boundaries with the host tuff, which probably represent more consolidated parts of the tuff. All of the pyroclastics examined and approximately 10% of the cumulate xenoliths contain carbonates in variable amounts, textures and mineralogy. In some of the tuff samples, dolomite occurs in large amounts (up to 57%), principally as immiscible globules in trachytic melt (now glass), and represents the first occurrence of carbonatite reported from the Massif Central. The other carbonates, magnesiosiderite in the mafic cumulates or occasionally in some tuffs, and calcite in the felsic cumulates, are always associated with a silicate glass of trachytic composition. Coexisting feldspars and carbonates in the various types of sample are approximately in Sr isotopic equilibrium with an initial ratio of about 07042. C- and O-isotopic compositions of the carbonates covary and cover a very wide range of composition fromˇ2 9t o 3 9 %(PDB) and from 87 to 245% (SMOW), respectively; the globular dolomites have primary igneous isotopic signatures and plot within the primary igneous carbonatite field. The combined C---O ---Sr isotopic data indicate that both the dolomitic carbonatitic and silicate magmas came from a mantle source with very similar isotopic characteristics. On compositional arguments, injection of a non-cogenetic carbonatite magma into a differentiating body of felsic magma within the crust is preferred to the unmixing of a relatively late, fractionated melt to carbonatitic and trachytic magmas. The textures, high d 13 Ca nd d 18 O values of Mg-siderite and calcite and their trend are consistent with post-magmatic precipitation of these carbonates, probably by interaction between a CO2---H 2O-bearing fluid (520 mol % CO2) undergoing Rayleigh distillation processes and the minerals and glass in the cumulates. Although minimum isotopic temperatures are 4100C for the calcites and 4275C for the magnesiosiderites, the isotopic data are compatible with the chemical, solvus temperatures of 4500C for the calcites and 4500---700C for the magnesiosiderites, if the CO2 content of the fluid decreased during the distillation processes. The highly variable K2O/ Na2O ratios of some of the glasses on the scale of a few microns may be another consequence of H2O---CO2 metasomatic processes. Explosive eruption of the two immiscible magmas that entrained the xenoliths probably occurred as a result of CO2---H 2O degassing processes soon after the arrival of the carbonatitic magma in the trachytic system.

Boron in central Andean ignimbrites: implications for crustal boron cycles in an active continental margin

Calc-alkaline pumices and lavas from the Neogene–Pleistocene central Andean Altiplano–Puna Volcanic Complex (APVC) contain quartz phenocrysts with abundant primary melt inclusions. Melt inclusions represent the melt composition of dacitic to rhyodacitic magmas prior to eruption and they are widely protected from eruptive degassing, low-temperature vapor-phase alteration and post-depositional contamination with boron-rich dusts and aerosols. The average boron isotopic compositions of melt inclusion and matrix glasses are uniform in the units studied (overall average δ 11B=−3.8±2.8‰, 1 SD) and overlap with the range of local basement rocks ( δ 11B=−11‰ to −5‰). B is enriched in glasses (55±14 ppm, 1 SD), whereas whole-rock contents range between 10 and 40 ppm. At similar B contents, incompatible trace element ratios like B/Nb are generally higher in the ignimbrites and lavas (B/Nb=2.4 and 4.0) than in crustally derived Paleozoic to Cretaceous granitoids from NW-Argentina (B/Nb=1.0–2.7). This suggests a minor contribution of high-B/Nb arc andesites and is consistent with petrogenetic models in which APVC dacites originated from hybridization of dominantly crustally derived melts with andesitic magmas. Boron-rich hydrothermal brines and salt-lake water from within the APVC have δ 11B=−4.1 to +5.7‰. These values are close to the APVC ignimbrite values and contrast with hydrothermal brines from the eastern Puna and eastern Cordillera, which have lower δ 11B=−10‰ to −17‰ and occur in an area of little volcanic cover. This regional pattern suggests that borate enrichments resulted from the leaching of local country rocks: ignimbrites in the W Altiplano–Puna and Paleozoic basement in the eastern Puna and eastern Cordillera.

Strontium isotopic signatures of the streams and lakes of Taylor Valley, Southern Victoria Land, Antarctica: Chemical weathering in a polar climate

We have collected and analyzed a series of water samples from three closed-basin lakes (Lakes Bonney, Fryxell, and Hoare) in Taylor Valley, Antarctica, and the streams that flow into them. In all three lakes, the hypolimnetic waters have different 87Sr/86Sr ratios than the surface waters, with the deep water of Lakes Fryxell and Hoare being less radiogenic than the surface waters. The opposite occurs in Lake Bonney. The Lake Fryxell isotopic ratios are lower than modern-day ocean water and most of the whole-rock ratios of the surrounding geologic materials. A conceivable source of Sr to the system could be either the Cenozoic volcanic rocks that make up a small portion of the till deposited in the valley during the Last Glacial Maximum or from marble derived from the local basement rocks. The more radiogenic ratios from Lake Bonney originate from ancient salt deposits that flow into the lake from Taylor Glacier and the weathering of minerals with more radiogenic Sr isotopic ratios within the tills. The Sr isotopic data from the streams and lakes of Taylor Valley strongly support the notion documented by previous investigators that chemical weathering has been, and is currently, a major process in determining the overall aquatic chemistry of these lakes in this polar desert environment.

Latest Cretaceous basin formation within the Salinian terrane of west-central California

Sedimentologic details provide new information about the controversial Late Cretaceous-early Tertiary history of the Salinian terrane, a displaced crustal block located in the California Coast Ranges. The preserved Upper Cretaceous strata were deposited as part of a fan-delta-submarine-fan system along the margin of a topographically steep basin bounded by normal faults. Coarse-grained sediments, derived from local basement rocks and volcanic rocks of a mature magmatic arc, were shed southward from a basement high north of the basin. Sediment compositions suggest connections to the southwestern Mojave Desert region. Paleomagnetic data collected from the central Salinian terrane indicate minimal northward transport relative to cratonal North America and support the hypothesis of an origin in southern California, rather than in southern Mexico/Central America. The Upper Cretaceous Salinian basin may have formed as an east-west-trending fore-arc graben that partitioned the Cordilleran arc into northern and southern segments, like the Sunda Strait, an extensional basin in Indonesia. Sunda Strait is a transition zone between Java, where the oceanic plate is subducting at right angles to the magmatic arc, and Sumatra, where the oceanic plate is subducting at an oblique angle. Like the right-lateral Central fault in Sumatra, right-lateral faults in the Sierra Nevada accommodated oblique subduction and facilitated extension where the subducting angle changed. The Late Cretaceous structures that accommodated extension in the Salinian terrane are not visible because of subsequent Cenozoic tectonism, which has slivered the terrane from a compact block into pieces that now extend from the Transverse Ranges to coastal locations north of San Francisco.

Nd isotope evidence for crust-mantle interaction in the generation of A-type granitoid suites in Labrador, Canada

Early Proterozoic “A-type” granite suites in Labrador, northeastern Canada, have initial Nd isotopic compositions that lie between those of a depleted mantle reservoir and the local basement rocks. One of these A-type granite suites consists of a linear array of discrete plutons that crosses the boundary between an Archean craton and an adjoining Early Proterozoic crustal block. The ϵ Nd of these granites (at formation, t = 1720 Ma) changes from −8 to +4 across this boundary, reflecting changes in the nature and antiquity of the continental crust. However, in both domains, the granites cannot be derived entirely by anatexis of the local basement rocks, and apparently require significant amounts of “juvenile” (mantle-derived) material. A-type granites have been widely interpreted as anatectic derivatives of lower-crustal rocks that were depleted by previous melt extraction. The Nd isotopic data suggest that these Labrador examples were instead mixtures of mantle-derived magmas and overlying lower-crustal rocks, and that new material may have been the volumetrically dominant component (50–90%) in most cases. As an alternative to the prevalent “residual-source” model, it is proposed that the distinctive geochemical characteristics of these A-type granites are a function of the high liquidus temperatures and primitive, Mg-rich character of subcrustal magma fluxes in post-orogenic and anorogenic environments, where such magmas are emplaced into stabilized continental crust. Further Nd isotope studies of other A-type granite suites are required to ascertain if models invoking crust-mantle interaction are more widely applicable.