Robert L. Cullers

The influence of sediment recycling and basement composition on evolution of mudrock chemistry in the southwestern United States

This paper reports systematic changes in mudrock composition through time on a single con- tinental cmstal block. The changes reflect both sediment recycling processes and changes through time in the composition of crystalline material being added to the sedimentary system and are related to tectonic evolution as the block matures from a series of accreted arc terranes to a stable craton. The major and trace element distributions reflect different aspects of the provenance of the mudrocks in this study. Major elements record sediment recycling processes as well as changing proportions of sedi- mentary and first-cycle source rocks. With the exceptions of KzO (which tends to increase), and SiOz and A&O3 (which show no trend), most major oxides tend to decline in relative abundance in younger mud- rocks. Patterns shown by the Index of Compositional Variability ( ( Fe03 + KrO + NaaO + CaO + MgO + MnO + TiOJIAlrO,) and by K20/A1203 indicate that the major oxide trends are due to decreasing proportions of nonclay silicate minerals and a concomitant increase in the proportion of clay minerals, probably due to decreasing input of first cycle detritus coupled with recycling of sedimentary material. Excursions from progressive trends, marked by increases in MgO, K20, and CaO, reflect episodes of large- scale input of nonclay first-cycle minerals from crystalline source rocks due to large-scale basement uplift. The chemistry of low-solubility trace elements, in contrast, is not sensitive to recycling effects and reflects the composition of first-cycle input. Incompatible elements are progressively enriched relative to compatible elements in younger mudrocks, and values for chondrite normalised rare earth elements also increase. In addition, the Eu anomaly becomes systematically more negative in younger samples. These trends cMnot be explained by diagenetic or weathering processes, and, therefore, indicate that the proportion of fnc- tionated granitic first-cycle detritus being added to the sedimentary system becomes greater with time. These results confirm the importance of tectonic setting in controlling mudrock chemistry, and also demonstrate that there is a dynamic relationship between the tectonic evolution of a continental block and the composition of its sedimentary mantle.

The geochemistry of shales, siltstones and sandstones of Pennsylvanian–Permian age, Colorado, USA: implications for provenance and metamorphic studies

A series of shales and sandstones found near the source of the Sangre de Cristo, Belden, and Maroon Formations from central Colorado were examined petrographically and were analyzed for major and selected trace elements, including the REE. The sandstones from the Belden Formation have higher quartz/feldspar ratios than do those from the Maroon and Sangre de Cristo Formations. Also, the alkali feldspar (i.e., orthoclase, microcline, perthite)/plagioclase ratio decreases in the order Sangre de Cristo Formation>Maroon Formation>Belden Formation, but the CIW′ (chemical index of weathering=molecular [Al2O3/(Al2O3+Na2O)]∗100) decreases in the order Belden Formation>Sangre de Cristo Formation>Maroon Formation. This suggests that the Belden Formation had a more plagioclase-rich granitoid source and more intense weathering of the source than did the Maroon and Sangre de Cristo Formations. Also, the variation in the elemental composition within the terrigenous sediment may be explained in terms of the variation in the observed minerals. Elemental ratios critical of provenance are statistically the same between the finer sediment of the Maroon and Sangre de Cristo Formations and fall within the range of a granitoid provenance, suggesting a similar granitoid source composition for the two formations. The fine sediment from the Belden Formation, however, has significantly more negative Eu anomalies and lower La/Sc and Th/Cr ratios than those of the Maroon and Sangre de Cristo Formations, suggesting a more differentiated granitoid source for the Belden than for the Maroon and Sangre de Cristo Formations. Most elemental concentrations or ratios vary by a factor of 0.12 to 60 between adjacent fine and coarse sediment (<1-m distances). Thus, it is not recommended that metasedimentary sequences similar in composition to this study be examined to determine element mobility during metamorphism as the variation due to sedimentary processes is so large.

The controls on the major and trace element variation of shales, siltstones, and sandstones of Pennsylvanian-Permian age from uplifted continental blocks in Colorado to platform sediment in Kansas, USA

Shales, siltstones, and sandstones of Pennsylvanian-Permian age from near the source in Colorado to those in the platform in eastern Colorado and Kansas have been analyzed for major elements and a number of trace elements, including the REEs. The near-source sandstones are significantly more enriched (Student t-test at better than the 99% confidence level) in SiO2 and Na2O concentrations and more depleted in Al2O3, Fe2O3 (total), TiO2, Th, Hf, Sc, Cr, Cs, REEs, Y, and Ni concentrations and LaCo and LaNi ratios than the near-source shales and siltstones, most likely due to more plagioclase and quartz and less clay minerals in the sandstones than in the shales and siltstones. There are no significant differences in K2O and Sr concentrations and EuEu∗, LaLu, LaSe, ThSc, ThCo, and CrTh ratios between the near-source sandstones and the near-source shales and siltstones. Samples of the Molas, Hermosa, and Cutler formations near the source that were formed in different environments in the same area contain no significant difference in EuEu∗, LaLu, LaSc, ThSc, ThCo, and CrTh ratios, so a generally silicic source and not the environment of deposition was most important in producing these elemental ratios. For example, CrTh ratios of near-source shales, siltstones, and sandstones range from 2.5 to 17.5 and EuEu∗ range from 0.48 to 0.78, which are in the range of sources of sediments derived from mainly silicic and not basic sources. Near-source shales and siltstones contain significantly higher (Student t-test) and more varied concentrations of most elements (Al2O3, Fe2O3, MnO, TiO2, Ba, Th, Hf, Ta, Co, Sc, REEs, Nb, Y) but significantly lower concentrations of Na2O and EuEu∗ than platform shales and siltstones in Kansas (e.g., La = 65.7 ± 40 and EuEu∗ = 0.55 ± 0.07 in near-source shales and siltstones and La = 23.7 ± 8.7 and EuEu∗ = 0.64 ± 0.08 in platform shales and siltstones). The SiO2 and CaO concentrations are not significantly different in platform shales and siltstones compared to the near-source shales and siltstones, so dilution of other minerals by quartz and calcite is not the main reason for the lower concentration of most elements in the platform relative to the near-source shales and siltstones. Rather the lesser concentrations of most elements in clay minerals of the platform shales and siltstones can account for the lower concentration of most elements compared to corresponding near-source shales and siltstones. The lower concentrations of many elements in clay minerals in the platform shales and siltstones may be a result of having been derived from recycling of clay minerals from older rocks. The greater homogeneity of elemental concentrations of the platform shales and siltstones compared to those in the source is also consistent with homogeneous mixing of such recycled material. Also there is no significant difference in ThSc, LaCo, ThCo, LaNi, and CrTh ratios of the near-source sedimentary rocks in Colorado to the platform shales and siltstones in Kansas, and the latter are also consistent with derivation from mostly silicic source rocks.

Geochemical signature of provenance in sand-size material in soils and stream sediments near the Tobacco Root batholith, Montana, U.S.A.

Abstract Trace-element geochemistry of sandstones are being used to determine provenance. We have conducted preliminary and limited experiments to determine to what extent daughter sands retain the geochemical signature of parent rocks. Six sets of first-order stream sediments, soils from adjacent slopes, and a variety of parent rocks were collected from southwestern Montana, U.S.A. Sampling in a low-relief area ensured that climate and residence time of soils on slopes could be eliminated as variables. Sand-size fractions of stream sediments and soils, and the corresponding parent rocks (granodiorite, quartz monzonite, granite gneiss, biotite-tonalite gneiss and amphibolite) were analyzed for most major elements and selected trace elements. Petrologic modal analysis of the parent rocks and the 0.25–0.50-mm fraction of each sand was done to monitor major mineralogic control, if any, on chemical compositions of the samples. Our data show that the abundances of the Si and Al in sediments do not discriminate provenance. Abundances of Ca, Mg, Fe and Ti may broadly distinguish between sands derived from metamorphic and igneous source rocks, at least in the area studied. Differences in abundances of the Ba and Th, and the ratio of La/Lu between granitic, tonalitic and amphibolitic parent rocks are preserved in the daughter sediments that we studied. However, the size of the Eu anomaly in the REE patterns of different daughter sediments is not diagnostic of parent rocks. Abundances of Co and Sc distinguish between sediments derived from felsic and mafic rocks. A better provenance discrimination is obtained if the ratios La/Sc, Th/Sc, La/Co, Ba/Sc and Ba/Co are used. Petrologic modal data show that mineral contents and chemical compositions of parent rocks are compatible with each other. The chemical composition of the sands may be roughly correlated to the petrological modal data but the abundances of some minor and trace elements of sediments cannot be inferred from modal mineralogy. This is expected because these elements may concentrate in accessory minerals and/or may weather out into aqueous or clay mineral fractions; it is also compatible with conclusions of previous studies that some of these elements do not reside in sand-size fractions of siliciclastic sediments.

Rare-earth element and mineralogic changes in Holocene soil and stream sediment: A case study in the Wet Mountains, Colorado, U.S.A.

Size-fractions of soil and stream sediment developed on plutonic, metamorphic, or mixed plutonic-metamorphic sources in the Wet Mountains, Colorado, have been analyzed for REE (rare-earth elements) and mineralogy. The clay-sized fractions (<0.5-μm and 0.5–2-μm, or <2 μm) have REE contents that are similar to more enriched than the source, Eu/Sm ratios that are similar to the source and La/Lu ratios that are higher than their corresponding source. The <0.5-μm fractions have lower REE contents than the corresponding 0.5–2-μm fraction in every stream sediment. For example, unweathered San Isabel batholith near the sampled stream sediment contains ΣREE = 511–712 ppm, and EuSmratio = 0.17–0.19 and (LaLu)cn = 9.8–10.3. The corresponding <0.5- and 0.5–2-μm fractions of sediment formed from San Isabel batholith are enriched in the REE (ΣREE = 1135 and 2225 ppm, respec.), are slightly lower in Eu/Sm ratios (0.16 and 0.14, respec.) and are higher in La/Lu ratios (12.2 and 16.3, respec.) than the unweathered rocks. The silts have LREE (light REE) contents and Eu/Sm ratios very similar to the source (due to similar mineralogy), but silts have HREE (heavy REE) contents higher than the source (due to high zircon). For example, silt-sized fractions in stream sediment draining the San Isabel batholith contain ΣREE = 808 ppm, EuSmratio = 0.18 and (LaLu)cn = 7.4. In contrast, the sands contain lower REE contents, similar to higher Eu/Sm ratios and lower La/Lu ratios than the corresponding source due to depletion in heavy minerals (concentrated in the REE and containing lower Eu/Sm ratios) relative to quartz and feldspar (depleted in the REE and containing higher Eu/Sm ratios). For example, the sand fraction from stream sediment from the San Isabel batholith has ΣREE = 206 ppm, EuSmratio = 0.17 and (LaLu)cn = 5.6. Small variations of accessory minerals concentrated in the REE like allanite or sphene may cause drastic variation in REE content in the sand fraction. Clays and silts derived only from the plutonic rocks have higher REE and similar to lower Eu/Sm ratios than corresponding fractions derived only from migmatites (e.g., ΣREE of silt fractions from migmatites = 224 ppm and from plutons = 375–808 ppm; Eu/Sm ratios from migmatites = 0.17 and from plutons = 0.11–0.18). The REE content and the Eu/Sm ratio of clays and silts derived from migmatites are much more similar to the average of MCPS (mid-continent platform sediment) than corresponding fractions derived from the plutons (MCPS has ΣREE = 192 ppm; EuSm = 0.19; LaLu = 8.0). The EuSm ratio of sands derived from plutons and migmatite are lower to mostly higher than the MCPS. The REE content of sands from plutons are not necessarily higher than the MCPS due to variation in heavy minerals like allanite or sphene concentrated in the REE. Sediments derived from mixed sources have REE contents in fine-fractions that may not necessarily reflect the REE content of the nearest source. For example, 0.5–2-μm and silt fractions developed on the San Isabel batholith with a migmatite upstream have the high REE content of the batholith (ΣREE in 0.5–2-μm fraction = 1100 ppm; silt = 573 ppm) while the corresponding <0.5-μm fraction has the very low REE content of the migmatites (ΣREE = 110 ppm). These results suggest fine-grained material near the source may sample a much larger area than coarser material near the source. Thus, analysis of fine-grained sediment associated with coarse-grained material near the source may give a much different impression of the source than would determination of the source using classic petrographic examination of the sand or gravels.

The controls on the major- and trace-element evolution of shales, siltstones and sandstones of Ordovician to tertiary age in the Wet Mountains region, Colorado, U.S.A.

A vertical section of shales, siltstones and sandstones from Ordovician to Eocene age in a limited geographic area east of the Wet Mountains and Sangre de Cristo Mountains in Colorado have been analyzed for major elements and a variety of trace-element concentrations, including the REE. In addition, the petrography of the sandstones has been determined. The quartz-rich arenites contain considerably lower concentrations of most elements relative to coexisting shales or siltstones at the same outcrop. The arkosic sandstones contain similar to lower concentrations of most elements compared to coexisting shales or siltstones in the same area. Accordingly, the average concentrations of Al2O3, Fe2O3, MgO, TiO2, LOI, Rb, Th, Co, Sc, Cr, Cs, Nb, Y and REE are significantly lower in all sandstones relative to all shales and siltstones. The exceptions are SiO2, Na2O and Ba concentrations, and the Eu/Eu★, LaSc and ThSc ratios as they are higher in the sandstones than the shales and siltstones. There is no significant difference in the MnO, CaO, K2O and Sr concentrations or the solLaCo, ThCo, LaNi, CrTh and (LaLucn ratios of the average sandstones relative to the shales and siltstones. Many of these differences may be related to the higher quartz and feldspar and lower clay mineral amounts in the sandstones than in the shales and siltstones. For example, average LaSc, ThSc, EuEu★ and (LaLucn ratios of th arkosic sandstones tend to be higher than the coexisting shales and siltstones due to enrichment of the arkoses in feldspar relative to the other minerals (e.g., EuEu★ = 0.82 ± 0.19 in average arkoses in the Trinidad, Vermejo, Raton, Poison Canyon and Cuchara Formations; EuEu★ = 0.63 ± 0.19 in average shales and siltstones in the same formations). In addition, the elemental concentrations and ratios are more variable for most elements in the sandstones than the shales and siltstones. This suggests that the shales are rapidly homogenized near the source. The elemental fractionation in the arkosic sandstones due to enrichment of feldspar relative to other minerals suggests that they are poorer indicators of provenance that the associated shales, whereas, the more homogeneous elemental distributions in the shales and siltstones suggest that they may be better indicators of provenance than the sandstones. Nevertheless, the elemental ratios that are most similar in the sandstones and in the shales and siltstones are consistent with their derivation from similar average intermediate to silicic source rocks.

Behavior of rare earth elements in a paleoweathering profile on granodiorite in the Front Range, Colorado, USA

A Paleoweathering profile on the Boulder granodiorite in northern Colorado provides an opportunity to trace the behavior of REEs from parent rock, through a weathering profile, into unconformably overlying Permian sediments. With progressive upward weathering of the granodiorite, Na2O, CaO, SiO2, TaHf, CoTh, CrSc, CrTh, ZrHf, LaSc, ZrY, and LaTh decrease; Al2O3 and Fe2O3T increase; and TiO2, MgO, K2O, P2O5, Rb, Zr, Sc, Cr, Co Hf, Nb, Ta, Y, Th, U, REE, TiNb, and ZrNb increase to maximum values and then either level off or decrease. LREE enrichment is less in the weathering profile than in the parent granodiorite and although the parent does not have an Eu anomaly (or only a slight positive anomaly), all samples from the weathering profile and overlying sediments have significant negative Eu anomalies. This observation is especially important in that it shows conclusively that a negative Eu anomaly can be produced during chemical weathering of granitoids. We suggest these Eu anomalies are due to relative enrichment of the other REEs and partial loss of Eu during the breakdown of plagioclase. The Boulder weathering profile also has a very minor negative Ce anomaly that is within error of a Ce anomaly in the parent. In the unweathered parent, >50% of the REE are contained in sphene, and in the case of La, also in allanite. From 10–20% of the REE are contained in apatite and biotite (± hornblende), and from 7–10% of the HREEs are in zircon. With exception of Eu, for which feldspars contribute about 8%, negligible amounts of REEs occur in the feldspars. In weathered samples, >75% of the REEs are contained in clay minerals. The crossover between sphene and clay control of REEs occurs over a distance of 1 m near the contact with fresh rock. Except for their small negative Eu anomalies, the clay minerals have REE patterns very similar to those of the parent rock. Isocon plots suggest apparent enrichments of many elements in the Boulder weathering profile result from losses of Na, Ca, and Si during plagioclase weathering. In addition, variable amounts of Sr, Eu, Ta, Nb, P, and Ba were lost during weathering. Although ThU, ZrY, ThSc, ZrHf, LuHf, and TiZr may have been transferred relatively unchanged from granodiorite parent to the bulk weathering profile, most other element ratios and REE distributions were significantly changed during weathering. This observation implies that caution needs to be exercised when using REE patterns and element ratios to trace sediment provenance. The fact that most element ratios and REE distributions also differ between Fountain sediments and the bulk weathering profile may be related to one or a combination of four factors, listed in order of probable decreasing importance: contribution of other sources to the Fountain sediments, sorting of minerals during sediment deposition, remobilization of elements during diagenesis, and leaching of elements by water flow through the upper meter of the weathering profile.

Geochemistry of the Mesoproterozoic Lakhanda shales in southeastern Yakutia, Russia: implications for mineralogical and provenance control, and recycling

Abstract Shales of the Lakhanda Group of Late Mezoproterozoic age (1050–1000 Ma) from the southeastern Siberian craton in Russia have been analyzed for major elements and a number of trace elements, including the REEs. Shales along the Maya River formed as platform sediments in a deeper shelf facies, whereas, shales along the Belaya River formed in more active and open environments of an upper shelf carbonate ramp. The log of most elemental compositions to Al2O3 ratios are the same in the Maya and Belaya River samples, suggesting a similar source rock composition for rocks in the two areas. The log of SiO2, MgO, Na2O, K2O, Rb, Ba, and Ni to Al2O3 ratios are significantly higher and the log of TiO2 to Al2O3 ratios are significantly lower in shales from the Belaya River than the Maya River sections. The CIA (chemical index of alteration) is thus significantly lower in the Belaya shales than the Maya shales, suggesting less weathering of the in the Belaya shales than the Maya shales. The ICVs (Index of Compositional Variability=Fe2O3+K2O+Na2O+CaO+MgO+TiO2/Al2O3) of the Lakhanda shales are less than 1, suggesting that they are compositionally mature and were likely dominated by recycling. Several samples have ICV>1, suggesting some first cycle input. The low K2O/Al2O3 ratios of these shales suggest that minimal first cycle alkali feldspar was present in the initial source. Most shales of the Lakhanda plot parallel and along the A–K line in A–CN–K plots suggestive of intense chemical weathering (high CIA) and do not indicate any clear-cut evidence of K-metasomatism or direct weathering back to the original source. If K-metasomatism produced these rocks, then they could have formed from tonalites to basalts. If weathering produced these rocks then they could have been produced from varied amounts of mostly granodiorite to granite. Elemental ratios critical of provenance (La/Sc, La/Cr, La/Co, Th/Sc, Th/Cr, Th/Co, and Eu/Eu*) are not significantly different between the Maya River and Belaya River shales, and the ratios are similar to fine-fractions derived from the weathering of mostly granitoids and not basic rocks. The Eu/Eu*, Th/Sc and low K2O/Al2O3 ratios of most shales suggest weathering from mostly a granodiorite source rather than a granite source, consistent with a source from old upper continental crust. Some samples at the bottom of the Belaya River section contain very low Eu/Eu* (0.35), suggesting significant input of first cycle detritus from highly differentiated granitoids similar to those from the Aldan Shield.

Rare-earths in size fractions and sedimentary rocks of Pennsylvanian-Permian age from the mid-continent of the U.S.A.

Abstract The REE (rare-earth) contents of sixty-three Lower Permian Sand and gravel-size fractions consist mostly of quartz or chert so their REE content is low (7.9–40.6 ppm) although heavy minerals may contribute a large fraction of the REE content. Unexpectedly, silt-size fractions have REE contents (74–355 ppm) that are usually lower but similar to their

Rare earth distributions in clay minerals and in the clay-sized fraction of the Lower Permian Havensville and Eskridge shales of Kansas and Oklahoma

The REE (rare earth element) content of a wide variety of clay mineral groups have been analyzed using radiochemical neutron activation and have been found to be quite variable in absolute REE content (range of ∑REE = 5.4–1732) and less variable in relative REE content (range of chondritenormalized La/Lu = 0.9–16.5). The variable REE content of the clay mineral groups is probably determined by the REE content of the source rock from which the clay mineral was derived and not from the separate minerals in the rock. The clay-sized fractions of the Havensville and Eskridge shales of Kansas and Oklahoma have similar relative REE distributions and identical negative Eu anomaly size as the composite of NAS (N. American shales), but an absolute REE content (range of ∑REE = 46–348) that may differ significantly from the composite of NAS. The clay-sized fraction of samples from any given outcrop did not vary much in absolute or relative REE content, but samples from northern Oklahoma, probably composed of continental to near-shore marine sediments, have higher absolute REE contents and higher La/Lu ratios than samples of marine deposits in Kansas (e.g. mean ∑REE in Oklahoma = 248; mean ∑REE in Kansas = 69–116). The differencess in the REE content between samples in Oklahoma and Kansas may be caused by chemical weathering processes in the source area, exchange reactions in the environment of deposition, or diagenesis and do not appear to be a result of the different clay minerals. Most samples have Eu anomalies relative to chondrites (range of Eu/Sm ratios of samples = 0.035–1.17; chondrites = 0.35). Some montmorillonites and kaolinites are anomalous in Eu relative to the NAS (range of Eu/Sm ratios of samples = 0.056–0.21; NAS = 0.22). These anomalies may be inherited from source rocks with Eu anomalies originally produced by igneous processes, or they may be produced by chemical weathering processes in the source area.