Provenance Discrimination Diagrams Research Articles

Geochemistry of metasedimentary rocks from the Puncoviscana Complex in the Mojotoro Range, NW Argentina: Implications for provenance and tectonic setting

Low-grade metasedimentary rocks from the Lower Paleozoic Puncoviscana Complex of the Mojotoro Range in NW Argentina have been analyzed for their major and trace elements - including rare earth - in order to constrain their provenance and depositional tectonic setting. They show a moderate degree of chemical weathering with intermediate CIA (<75) values and can be classified as metawackes and metapelites. Provenance discrimination diagrams based on major element geochemistry and trace element ratios (i.e., Cr/V, Y/Ni, Th/Co, La/Sc, Th/Sc, Eu/Eu*) suggest that the sediments were derived from upper crustal, felsic source rocks. Tectonic setting discrimination diagrams based on major elements, immobile trace elements (i.e., La- Th- Sc and Th-Sc-Zr/10) and La/Yb vs. Eu/Eu* plots, support an active margin setting associated with a magmatic arc located to the east of the Neoproterozoic–Cambrian Pampean Brazilian arc that formed along the margin of a Mesoproterozoic craton.

Geochemical characteristics of stream sediments from an urban-volcanic zone, Central Mexico: Natural and man-made inputs

Geochemical characteristics of stream sediments [n=31; Upstream section: Zahuapan River (1–12) and Atoyac River (13–20); Downstream section (21–31)] from Atoyac River basin of Central Mexico have been evaluated. The study focuses on the textural, petrography and chemical composition of the fluvial sediments with the aim of analyzing their provenance, the chemical weathering signature and their potential environmental effects. The fluvial sediments are mostly composed of sand and silt sized particles dominated by plagioclase, pyroxenes, amphiboles, K-feldspar, biotite, opaque and quartz. The sediments were analyzed for determination of major (Al, Fe, Ca, Mg, Na, K, P, Si, Ti), trace elements (As, Ba, Be, Co, Cr, Cu, Mo, Mn, Ni, Pb, Sc, V, Y, Zn, Zr, Ga) and compared with Upper continental crust (UCC), source area composition and local background values. The elemental concentrations were comparable with the average andesite and dacitic composition of the source area and the local background values except for enrichment of Cu (56.27ppm), Pb (34ppm) and Zn (235.64ppm) in the downstream sediments suggesting a significant external influence (anthropogenic). The fluvial sediments of Atoyac River basin display low CIA and PIA values implying predominantly weak to moderate weathering conditions in the source region. Based on the provenance discrimination diagrams and elemental ratios, it is understood that the collected sediments are derived from intermediate to felsic volcanic rocks dominated in the study region. Metal contamination indices highlight the enrichment of Cu, Pb, Zn, Mo, Cr and S clearly indicating the influences from natural (weathering and volcanic activity) and external (anthropogenic) sources. Ecological risk assessment results indicate that Cr, Ni and Zn will cause adverse biological effects to the riverine environment.

Late Quaternary loess-like paleosols and pedocomplexes, geochemistry, provenance and source area weathering, Manasbal, Kashmir Valley, India

The late Quaternary loess and loess-like deposits in Kashmir Valley are natural archives that have preserved paleoclimate and paleoenvironmental records of the region. We present a loess-like paleosol located along the margin of the Manasbal Lake, Ganderbal, which was studied in detail for understanding the pedological processes and reconstructing the late Quaternary soil formation. In this paper we present loess-like paleosol formation of a nearly 10.6m thick sequence exposed along the margin of Manasbal Lake, Ganderbal District, Srinagar, Kashmir. Geochemical and textural data of this loess-like sedimentary sequence fluctuate reflecting the varied depositional processes operating in the valley, differential intensity of weathering, and processes of pedogenesis. Weathering indices such as chemical index of alteration, chemical index of weathering, and plagioclase index of alteration reveal weak to moderate weathering of the parent material. Provenance discrimination diagrams of the present study disclose that the Manasbal loess-like paleosol sediments are derived from the mixed source rocks suggesting a variety of provenance with variable geological settings, which apparently have undergone weak to moderate recycling processes. The Manasbal paleosol horizons have been dated by the optically stimulated luminescence (OSL) method to the marine isotope stages mid-MIS-3 (41.7±8.0ka) and late-MIS-2 (14.6±3.8ka). During the MIS-3 period, the climate was wetter, forming a strong AhBtk paleosol as inferred from the geochemical data. A steady increase in the CaCO3 content and C/N ratio in the paleosols from 6.50m (MIS-3) indicates arid and drier climatic conditions. The area around Manasbal Lake incised because of climate change and neotectonic activity since post-14ka.

Geochemical evolution of Oligocene–Middle Miocene sediments in the deep-water area of the Pearl River Mouth Basin, northern South China Sea

The deep-water area of the Pearl River Mouth Basin in the South China Sea has received much scientific attention since the Ocean Drilling Program (ODP) Leg 184 in 1999 due to its potential economic prospects and distinct tectono-sedimentary evolutionary processes. In this study, we present the composition of major and trace elements from two newly sampled deep-water boreholes (BY6 and LW3) in the Baiyun Sag of the southern Pearl River Mouth Basin. The geochemical evolution in the Oligocene–Middle Miocene, as well as potential controlling factors, are investigated based on a comparative study with previous data from ODP site 1148 and borehole PY33. The Chemical Index of Alteration (CIA) and A–CN–K plot reveal that the observed weathering trends are not compatible for the four discussed boreholes. Sedimentary sorting is primarily observed in borehole PY33, where data trend away from the A apex to the feldspar join in the A–CN–K plot and show a spread of Al2O3/SiO2 and Nb/Zr ratios. Compared to chemical weathering and hydrodynamic sorting, provenance has a greater impact on sediment composition of the deep-water area. From the north, the Pearl River was the primary sediment supply. However, a positive Eu anomaly and the provenance discrimination diagrams (i.e., La/Th versus Th/Yb and Zr/Co versus Th/Co) reveal the mafic nature of borehole BY6 sediments in the Zhuhai–Lower Zhujiang (32.0–18.5 Ma) and Upper Hanjiang (13.8–10.5 Ma) formations. These compositions are unusual and differ from the well-defined felsic sources in the majority of the Baiyun Sag; these discrepancies are likely related to multistage magmatism. The sediments at site 1148 are characterized by slightly enriched heavy rare earth elements and relatively high Zr/Co ratios, which could possibly be caused by zircon enrichment from local sources.

Provenance and tectonic setting of Miocene siliciclastic sediments, Sibuti formation, northwestern Borneo

Provenance and tectonic setting of sandstone and mudstone units of the Miocene Sibuti Formation from northwest Borneo have been studied based on the mineralogy, major and trace element geochemistry data. The X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) data revealed that the sandstones and mudstones were abundant in quartz, pyrite, clay, and heavy minerals such as zircon, rutile, and some detrital cassiterite. Geochemically, the sandstones and mudstones are classified into quartz arenite, litharenite, sublitharenite, arkose, and wacke. Quartz arenites are enriched with SiO2, Zr, and Th and depleted in Al2O3, CaO, and other elements compared to other sandstone types, indicating high maturity and intensive weathering. Chemical index of alteration (CIA: 77–90), plagioclase index of alteration (PIA: 86–100), and A-CN-K diagram suggest intense weathering in the source area. Elemental ratios such as La/Sc, Th/Sc, Cr/Th, La/Co, and Th/Co are similar to sediments derived from the felsic rocks. Also, the provenance discrimination diagrams suggest recycled continental nature of these clastic sediments which are mostly derived from metasedimentary source (Rajang Formation). Discriminant-function diagram for the tectonic discrimination of siliciclastic sediments revealed that the sediments of Sibuti Formation were derived from a collision zone, which is consistent with the geology of the study area.

Geochemistry of the Late Jurassic–Early Cretaceous shales (Shurijeh Formation) in the intracontinental Kopet-Dagh Basin, northeastern Iran: implication for provenance, source weathering, and paleoenvironments

In the present study, the geochemical data were used to investigate the provenance, weathering history, and paleoenvironmental studies of the Shurijeh Shales, Kopet-Dagh Basin. The X-ray diffraction and X-ray fluorescence spectrometry techniques are used for geochemical analyses. The provenance discrimination equations and plots of the Shurijeh Shales in the provenance discrimination diagrams indicate that the nature of the source rocks probably was acidic (granitic). With attention to the chemical index of alteration, plagioclase index of alteration, and chemical index of weathering values which range from 76.26 to 95.48 %, it can be contended that the litho-components of the studied shales were subjected to intense chemical weathering. The low values of Ni/Co and Cu/Zn ratios and investigation of P2O5, V, and Al2O3 values of Shurijeh Shales suggest that the deposition of these shales is performed under oxic shallow marine to fluvial environments.

Geochemistry of loess-paleosol sediments of Kashmir Valley, India: Provenance and weathering

Middle to Late Pleistocene loess-paleosol sediments of Kashmir Valley, India, were analyzed for major, trace and REE elements in order to determine their chemical composition, provenance and intensity of palaeo-weathering of the source rocks. These sediments are generally enriched with Fe2O3, MgO, MnO, TiO2, Y, Ni, Cu, Zn, Th, U, Sc, V and Co while contents of SiO2, K2O, Na2O, P2O5, Sr, Nb and Hf are lower than the UCC. Chondrite normalized REE patterns are characterized by moderate enrichment of LREEs, relatively flat HREE pattern (GdCN/YbCN=1.93–2.31) and lack of prominent negative Eu anomaly (Eu/Eu*=0.73–1.01, average=0.81). PAAS normalized REE are characterized by slightly higher LREE, depleted HREE and positive Eu anomaly. Various provenance discrimination diagrams reveal that the Kashmir Loess-Paleosol sediments are derived from the mixed source rocks suggesting large provenance with variable geological settings, which apparently have undergone weak to moderate recycling processes. Weathering indices such as CIA, CIW and PIA values (71.87, 83.83 and 80.57 respectively) and A-CN-K diagram imply weak to moderate weathering of the source material.

Provenance evolution of collapse graben fill in the Himalaya—The Miocene to Quaternary Thakkhola-Mustang Graben (Nepal)

This study focuses on the detailed provenance evolution of young, syn- to post-orogenic extensional grabens in orogens like the Himalaya to trace the tectonic history of such late-stage basins, using the Neogene Thakkhola-Mustang Graben as a case study. The graben is situated within the Tibetan-Tethys zone and is filled with > 870 m of continental deposits of Miocene to Holocene age-. Based on logged sections within the predominantly alluvial to coarse-grained fluvial fill of the graben we investigated paleocurrent data and the petrology of sandstones and conglomerates including heavy minerals studies to interpret provenance and source areas in detail. Significant changes are recorded by slight differences in heavy mineral and pebble compositions. The sandstones can be classified as lithic greywackes, lithic arkoses and feldspathic litharenites. Sandstone, mudstone, quartzite and some granite clasts are dominant in conglomerates of the central part of the graben. Tetang Formation conglomerates of Miocene age comprise mostly clasts of Mesozoic rocks with an eastern provenance, consistent with measured paleocurrent directions. All paleocurrent data and compositional analyses of imbricated conglomerates of the Miocene–Pliocene Thakkhola Formation in the northeast of the graben suggest that clasts were derived from eastern source areas comprising mainly Mesozoic rocks whereas Paleozoic clasts of a western to northern source area predominate in the centre of the graben. Heavy mineral analysis indicates that tourmaline, staurolite, zircon, garnet and apatite constitute a significant proportion of the assemblages of all formations through time whereas epidote, andalusite, kyanite, chloritoid, hornblende, chrome-spinel, rutile and amphiboles are less common. These assemblages reflect in general stable minerals and low to high-grade metamorphic source rocks, and are principally controlled by reworking of older, passive margin sediments of the Tibetan-Tethys zone as indicated by provenance discrimination diagrams. Three successive stages in provenance evolution were recognized: (1) The Miocene Tetang Formation, characterized by higher kyanite values, corresponding to the Himalayan foreland evolution; (2) the Thakkhola Formation, characterized by granite clasts and significantly higher amounts of andalusite, indicating source area expansion and erosion of the Mustang-Mugu granites to the northwest; (3) the Upper Pleistocene/Holocene Kaligandaki Formation, bearing higher amounts of epidote/klinozoisite and ophiolite and high-pressure/low temperature detritus as indicated by chrome spinel and blue amphiboles, derived from the north-lying Indus-Tsangpo suture zone. The change in source areas from the Miocene/Pliocene to the Late Pleistocene/Holocene is interpreted as a result of the evolution from an initial stage of high-angle normal faulting and collapse basin formation to a low-angle extensional detachment basin system.

Sand composition in an Iberian passive-margin fluvial course: the Tajo River

The Tajo River, the 10th largest river in Europe, drains part of the western passive margin of Europe that includes multiple tectonic elements of the Iberian plate. Modern fluvial sand composition in the Tajo River drainage basin reflects the nature of the source region, which lies in the central part of the Iberian Peninsula. Four fluvial petrographic provinces (A, B, C and D) can be established in the Tajo drainage basin, corresponding well with the four principal structural units drained: (1) the Iberian Range; (2) the Hesperian massif; (3) The Tertiary Tajo basin; and (4) the Neogene Santarem–Lisboa basin. Province A corresponds to the Tajo River head and is characterized by quartzolithic sedimenticlastic sands (Qm 67F 4Lt 29 and Rs 79Rg 3Rm 18). These sands have been derived from diverse Mesozoic siliciclastics and carbonates of the Iberian Range. Province B appears in the upper reaches of the Tajo River course and is quartzofeldspathic (Qm 57F 34Lt 9) with diverse rock fragments (Rs 34Rg 23Rm 43). Sources are Hercynian granitoids and metasediments and Neogene clastics and minor carbonates of the Tertiary Tajo basin. Province C extends along the middle course of the Tajo River with quartzofeldspathic metamorphiclastic sand modes (Qm 60F 33Lt 7; Rs 4Rg 21Rm 75). The sources are metamorphic rocks intruded by plutonites of the Hesperian Massif. Province D is quartzofeldspathic (Qm 55F 39Lt 6) with a dominance of phaneritic rock fragments (Rs 5Rg 33Rm 62) and corresponds to the lower reaches of the Tajo River, where siliciclastic deposits of the Neogene Santarem–Lisboa basin are the main sources. Sands plot on provenance-discrimination diagram (QmFLt) within the recycled-orogen field (Tajo River head) and continental-block fields (upper, middle and lower course). In addition, we have proven the usefulness of the RsRgRm diagram to discriminate the defined fluvial provinces, originating from heterogeneous parent-rock textures and mineralogy. Climate does not exert any strong influence on the petrogenesis of the Tajo River drainage basin sand, and erosion in the source areas may be described in terms of weathering-limited denudation regime. By contrast, mixing with tributary supplies is the main process that modifies composition in the Tajo River sand. The establishment of fluvial provinces related to the main bedrock structural units reflects the great relevance of tributaries from each province in the generation of the Tajo River sand and the low significance of inherited sandy load from previous provinces. The abundance of granitoid rock fragments (RsRgRm%Rg) in fluvial sand of both tributaries and main channel of the Tajo River drainage basin faithfully represents the relative abundance of granodiorite+monzogranite bedrock exposure in each subbasin. Metasedimentary and metamorphic bedrock outcrop area is overrepresented by metamorphic rock fragments (RsRgRm%Rm) in both sand from tributaries and from the Tajo main trunk river. This is manifested by differences of 18–23% between means of metasedimentary outcrop area and the RsRgRm%Rm mean in the sand from Provinces B and C+D, respectively. Sedimentary (mainly carbonate) outcrop area is underrepresented or not represented by sedimentary rock fragments (RsRgRm%Rs) in sand of the Tajo River. This underrepresentation causes a difference between means of sedimentary outcrop area and the RsRgRm%Rs mean of 21%, 17.1% and 10.7% in sands from Province A, B and C+D, respectively. This fact confirms the rapid loss of these grains during transport because of their labile nature.

Depositional environments of Lower Pennsylvanian coal-bearing siliciclastics of southeastern Tennessee, northwestern Georgia, and northeastern Alabama, U.S.A.

In the Cumberland Plateaus of southeastern Tennessee, northwestern Georgia, and northeast Alabama, Pennsylvanian strata are siliciclastics containing discontinuous coal seams. Above some of these coal seams, the shale deposits contain fossils of marine or brackish fauna. The entire sequence was deposited in an asymmetric foreland basin and is thickest in the southeast. Within this general trend there are locally thick deposits of lowermost Pennsylvanian Gizzard Group sequences that mark subbasins. Conglomeratic sandstone members of the Gizzard Group are discontinuous and tend to be thicker in the subbasins. In contrast, conglomeratic to sandy units are more laterally continuous in the overlying Crab Orchard Mountains Group. The Pennsylvanian sequence overlies paleosols with subjacent freshwater-neomorphosed shallow marine carbonates, or siliciclastics of the uppermost Mississippian Pennington Formation. A provenance discrimination diagram indicates that Pennsylvanian siliciclastics were derived from an orogenic source. Profile analysis of thick quartzose sandstone sequences indicates facies, architectural elements and bounding surfaces characteristic of braided stream deposits. A dominant southwest paleoflow direction is inferred from paleocurrent indicators in sandy braided and meandering stream deposits.

The Provenance of Middle Tertiary Sandstones of the San Emigdio Range and Tehachapi Range, Southern California: ABSTRACT

Eocene to lower Miocene sandstones and shales in the San Emigdio Range of southern California were deposited in fan deltas and braid deltas along the tectonically active southeastern margin of the San Joaquin basin. The sandstones are arkoses and lithic arkoses. Framework grain s include quartz, K-feldspar, plagioclase, and volcanic/ metavolcanic, metamorphic, and sedimentary/ metasedimentary lithic fragments. Modal point-count data plot within the uplifted basement and dissected magmatic arc fields on provenance discrimination diagrams, suggesting dissected Sierran and Mojave block sources. ^Nd(t) values of -7.85 and -6.68 from two sandstone samples in the Tejon Formation (Eocene) also suggest a Mojave block source. Modal averages show distinct differences between sandstones deposited in various subenvironments along the basin margin. In general, eolian (QFL=80,16,4) and beach (QFL=75,13,12) sandstones are more mature than fluvial (QFL=40,47,13), nearshore (QFL=53,40,7), Gilbert delta (QFL=49,39,12), and shelf sandstones (QFL=41,46,13). The relative immaturity of the shelf sandstones probably is a result of sand bypassing from nearshore and deltaic settings onto the shelf during storms. Climate does not appear to have exerted a major control on sandstone composition. Quartz-rich sandstone in the Uvas Conglomerate (lower Eocene) and the upper San Emigdio Formation (upper Eocene) may have resulted in part from hot, humid climate. Feldspar-rich sandstone in the Pleito Formation (lower Oligocene) may reflect climatic deterioration at the Eocene-Oligocene boundary.

Composition of Holocene Colorado River Sand: An Example of Mixed-Provenance Sand Derived from Multiple Tectonic Elements of the Cordilleran Continental Margin

ABSTRACT The Colorado River and its tributaries drain a complex provenance that includes multiple tectonic elements of the Proterozoic to Phanerozoic Cordilleran margin. In order to provide (1) a more reliable estimate than that reported in the literature for the composition of Holocene Colorado River sand, and (2) a standard for comparison with ancient sandstones that may contain detritus derived from a similar complex provenance, 25 unconsolidated samples were collected along the lower reaches of the Colorado River for detailed petrological examination. Each specimen was artificially consolidated with epoxy, thin sectioned, and stained for feldspar. Each author, utilizing the Gazzi-Dickinson point-count method, counted 400 points per sample; the results from the two point-count operators wer averaged. The average Colorado River sand in our collection is medium-grained, moderately to well sorted, and composed of 66.2%Q, 18.1%F, 14.7%L, and 1.3% miscellaneous grains. Q, on average, is composed of 89% monocrystalline and 11% polycrystalline quartz grains. The average plagioclase-to-total-feldspar ratio is 0.66, and the average unstable-aphanitic-rock-fragment population (L) consists of 6% metamorphic (Lm), 43% volcanic (Lv), 51% sedimentary (Ls), and 1% alterite fragments. On the QFL provenance-discrimination diagram developed by W. R. Dickinson and associates, our point-count data plot entirely within the recycled-orogen field. W. R. Dickinson and colleagues ascribed such provenances to subduction complexes, collision orogens, and foreland uplifts ass ciated with fold-and-thrust belts. Rocks in the Colorado River drainage basin, however, include (1) remnants of Precambrian crystalline rocks which form a part of cratonal North America; (2) miogeoclinal and platformal sequences that developed along the North American continental margin during Paleozoic time; (3) deformed Paleozoic and Mesozoic sedimentary successions that make up the Sevier-Laramide fold-and-thrust belt; (4) Mesozoic and Cenozoic sedimentary fill of the Sevier foreland basin; (5) Cenozoic basement-cored uplifts that formed during the Laramide orogeny; (6) plutonic and volcanic rocks of the Mesozoic and Cenozoic Cordilleran continental-margin magmatic arc; and (7) Cenozoic sedimentary and volcanic rocks associated with extension and dextral shear along the Cordilleran co tinental margin. Thus, the QFL compositions of Holocene sand derived from the Colorado River drainage basin do not fit neatly into the recycled-orogen model proposed by W. R. Dickinson and colleagues. In contrast, on the QmFLt provenance-discrimination diagram also engendered by W. R. Dickinson and associates, 16 samples plot in the mixed-provenance field, and nine specimens plot in the quartzose-recycled or transitional-continental fields. On the LmLvLs diagram formulated by R. V. Ingersoll and C. A. Suczek, our samples plot close to the Lv-Ls join, approximately midway between the fields of magmatic arcs, rifted continental margins, and suture belts, probably as a result of their mixed heritage. Thus, the QmFL SUB>t and LmLvLs diagrams produce results that reflect more accurately, than do the results derived from the QFL provenance-discrimination diagram, the complex multiple tectonic characteristics of the source rocks in the Colorado River drainage basin. Our samples, therefore, represent excellent examples of mixed-provenance sands. Casual use of only the QFL diagram can lead to erroneous interpretations of provenances composed of more than one major tectonic element.

Petrology of Holocene Sand, Peninsular Ranges, California and Baja Norte, Mexico: Implications for Provenance-Discrimination Models

ABSTRACT The Peninsular Ranges of southern California and Baja California Norte, Mexico, represent an uplifted and variably dissected Mesozoic magmatic arc. They are composed mostly of Cretaceous plutonic rocks and Mesozoic greenschist to amphibolite facies metasedimentary and metavolcanic rocks. In order to evaluate the reliability of the diamond-shaped QnuQuQp provenance-discrimination diagram developed by Basu et al. (1975) and the QFL provenance-discrimination diagram developed by Dickinson et al. (1983), we collected 62 samples of Holocene sand from four drainage basins along the west side of the Peninsular Ranges. The QFL composition of each specimen was determined by the Gazzi-Dickinson point-counting method; the relative percentages of med um-grained fragments of Qnu, Qu, and Qp were determined by counting 100 grains in each specimen. The results of our study indicate that Holocene sands derived from the Peninsular Ranges have the following average petrological characteristics: 1) Sands derived mostly or exclusively from plutonic rocks are composed of 16-46 percent Q, 54-83 percent F, and 0.2-0.5 percent L. The medium-grained quartz population produced by erosion of mostly or exclusively plutonic rocks consists of 61-77 percent Qnu, 4-17 percent Qu, and 13-27 percent Qp. 2) Sands originating from the erosion of metasedimentary rocks are composed of 63 percent Q, 13 percent F, and 24 percent L. The medium-grained quartz population derived from a metasedimentary source is made up of 25 percent Qnu, 9 percent Qu, and 66 percent Qp. 3) Sands produced by the erosion of mostly or exclusively metavolcanic rocks are composed of 19 percent Q, 39-44 percent F, and 38-43 percent L. The medium-grained quartz population originating from a mostly or exclusively metavolcanic source consists of 48-74 percent Qnu, 6 percent Qu, and 20-46 percent Qp. Petrological data obtained during this study indicate that the QFL provenance-discrimination model generally yields a correct interpretation of tectonic setting when applied to Holocene sand from the Peninsular Ranges. Data, however, also show that local source areas produce sands that, when interpreted on the basis of their position on the QFL provenance-discrimination diagram, yield an erroneous interpretation of tectonic setting. Data obtained from counting varieties of medium-grained quartz show that detrital quartz populations produced by plutonic rocks are discriminated from those derived from metasedimentary rocks by the QnuQuQp provenance-discrimination diagram. In contrast, quartz populations originating from a mostly or exclusively metavolcanic source are difficult to interpret on the QnuQuQp diagram. Therefore, we recommend that the QFL provenance-discrimination model be used with caution when sand or sandstone samples are from a limited geographic area and that the QnuQuQp provenance-discrimination diagram be used with prudence when metavolcanics are suspected in the provenance.

Petrology and Provenance of Upper Cretaceous Sandstone, Southern San Rafael Mountains, Santa Barbara County, California: ABSTRACT

Petrologic analysis of 24 medium to coarse-grained sandstone samples, collected from a 2950-m submarine fan complex of late Campanian-early Maestrichtian age exposed within Mono Creek Canyon, reveal commonly calcite cemented, poorly sorted, subangular biotic arkoses. Framework averages 86.0%. Matrix - primarily detrital quartz, feldspar, and lithic fragments finer than 0.03 mm and mechanically and chemically altered phyllosilicates and labile aphanites - averages 8.9%. Calcite cement averages 4.2%. Porosity averages 0.9%. Gazzi-Dickinson point counts of 400 framework grains per slide yield modal averages of Q/sub 37.7/ F/sub 49.8/ L/sub 12.5/; Qm/sub 27.4/ F/sub 49.8/ Lt/sub 22.8/; Qm/sub 35.6/ P/sub 43.7/ K/sub 20.7/; and Qp/sub 49.4/ Lv/sub 22.1/ Ls/sub 28.5/. P/F averages 0.68, Lv/L averages 0.45, Qp/Q averages 0.27, and detrital phyllosilicate, predominantly biotite, averages 5.7% of total framework. Neither primary nor secondary parameters vary systematically with stratigraphic position. Miscellaneous constituents average 1.3% of framework and include epidote, garnet, amphibole, pyroxene, zircon, and tourmaline as well as carbonaceous blebs, opaque minerals, and unidentifiable lithic fragments. Separate analysis of 100 medium sized quartz grains per slide indicates a mean population of 63.0% non-undulatory monocrystalline quartz, 9.1% undulatory monocrystalline quartz, 10.1% polycrystalline quartz of 2 to 3 crystals, and 17.9% polycrystalline quartz composed ofmore » more than 3 crystals. Modal data, plotted upon provenance discrimination diagrams, indicate a plutonic provenance transitional between a dissected magmatic arc and uplifted basement terrane. Paleocurrent data, neglecting possible clockwise rotation, indicate sediment transport from the north.« less

Provenance of sandstone inclusions in the Paleozoic Sierra City mélange, Sierra Nevada, California

The Sierra City melange between Bowman Lake and East Fork Creek contains abundant feldspar-poor and feldspar-rich sandstone inclusions. A petrologic study of samples from these inclusions indicates that the average feldspar-poor sandstone is composed of 69% Q, 4% F, and 27% L. In contrast, the average feldspar-rich sandstone is composed of 34% Q, 46% F, and 20% L. On the QFL provenance-discrimination diagram of Dickinson and others, the feldspar-rich sandstones plot in the dissected magmatic-arc field, and the feldspar-poor sandstones plot in the recycled-orogen field. Previous work indicates that detrital zircons in the plagioclase-rich sandstones were derived from a source that probably included Upper Cambrian/Lower Ordovician rocks and that feldspar-poor sandstones contain round to subround Precambrian detrital zircons. These data indicate that feldspar-rich sandstones were derived from lower Paleozoic volcanic/plutonic rocks and that feldspar-poor sandstones probably were derived from associated metasedimentary, sedimentary, and volcanic rocks. The location of the source of the sand-sized feldspathic and volcanic detritus in the Sierra City melange is problematical. The source may be an unidentified magmatic-arc terrane located somewhere along the Pacific rim.