High CIA Research Articles

Geochemistry of black shales from the Mesoproterozoic Srisailam Formation, Cuddapah basin, India: Implications for provenance, palaeoweathering, tectonics, and timing of Columbia breakup

The Mesoproterozoic Srisailam Formation, exposed along the northern part of the Cuddapah basin, India, comprises mainly medium- to fine-grained siliciclastics, and is devoid of any carbonate sediment. Preliminary sedimentological studies helped in recognizing fifteen distinct facies (five facies associations) in Chitrial outlier of the Srisailam Formation deposited in continental half-graben basin(s). Black shales (sensu lato) are minor components of the Srisailam Formation, and inferred to have deposited in deep lacustrine and prodelta facies of the half-graben(s). The black shales show restricted thickness (up to 29.0m), and are characterized by overall high ‘black shale' to ‘total shale' ratio (>0.51). Their geochemical characteristics were studied to constrain provenance, palaeoclimate, and tectonic setting of deposition of the Srisailam Formation. Further, an attempt has been made to use the Srisailam black shales as proxy for constraining the timing of breakup of the supercontinent Columbia.The Srisailam black shales are geochemically quite distinct. At similar SiO2 contents they are considerably different from PAAS. They are characterized by considerably lower ΣREE (Av. 136.0±50.4ppm) but a more conspicuous negative Eu-anomaly (Av. 0.34±0.09) than PAAS. Al2O3/TiO2 and TiO2/Zr ratios coupled with Eu/Eu*, GdCN/YbCN, La/Sc, Th/Sc, and Th/Cr ratios suggest their derivation from granite and granodiorite. The CIA values (65–90, Av. 72±9) as a whole indicate moderate chemical weathering under semiarid climate. Discriminating geochemical parameters indicate passive margin depositional setting. The combined sedimentological and geochemical characteristics reveal deposition of the Srisailam sediments in continental rift basin(s).Thick succession of black shales (with high CIA values) that deposited with shelf carbonates proxy for mantle superplume and supercontinent breakup events. The sedimentological characteristics and geochemical data of the Srisailam black shales plausibly exclude any large-scale breakup of Columbia during the interval (1400–1327Ma) of deposition of the Srisailam Formation.

Provenance and paleo–weathering of Tertiary accretionary prism–forearc sedimentary deposits of the Andaman Archipelago, India

In order to understand the provenance and tectono–sedimentary processes occurring in the Andaman Subduction Zone (ASZ), the Late Cretaceous to Oligocene sedimentary records from the Andaman Islands have been studied. These sedimentary records are considered to have preserved the history of the India–Asia collision, evolution of the Himalayas, climatic development and palaeo–drainage reorganizations on the Indian and Asian plates. About 47 sandstones and mudstones (shales and siltstones) samples were analyzed for whole rock major, trace, and rare earth element compositions. The geochemical results suggest mixing of sediments derived from the mafic igneous sources comprising local ophiolites and volcanic arc of the ASZ and an older Archean to Proterozoic age felsic cratonic source with compositions similar to average granodiorite or upper continental crustal sources. The compositions were dominated by sources of the mafic arc during deposition of the Mithakhari Group, whereas they were controlled by continental sources during deposition of the Andaman Flysch Group. The Hope Town Conglomerate unit of the Mithakhari Group was mainly derived from weathering and erosion of the subaerially exposed local ophiolite thrust sheets, whereas its Namunagarh unit contains significant detritus from volcanic arcs. The Andaman Flysch turbidites were deposited with a greater supply of sediments from first–cycle active continental margin sources probably located in the Tibetan and eastern Myanmar region and recycled quartzose sedimentary sources within the nascent Himalayas. The sediments supplied to both the Mithakhari and the Andaman Flysch Groups were characterized by varying values of CIA, PIA and W. These variable values were either due to non–steady state weathering conditions in the sources or the changing climatic conditions owing to the motion of Indian plate with reference to the equator. The uniformly high CIA and W values in the Andaman Flysch rocks can be related to high precipitation and strong chemical weathering associated with the initiation of the Indian monsoon.

Age, provenance and tectonic setting of Neoproterozoic to early Paleozoic sequences in southeastern South China Block: Constraints on its linkage to western Australia-East Antarctica

Zircon U-Pb-Hf isotopes and whole-rock geochemical data are applied to investigate the age, provenance and tectonic setting of the late Neoproterozoic and overlying sequences from the Cathaysia Block of South China, in order to understand the tectonothermal evolution of this block and its linkage to supercontinents. New zircon U-Pb ages, in combination with previous fossil data, indicate the lower and upper Louziba Groups were deposited between 655 and 635Ma (Cryogenian) and 635 and 542Ma (Ediacaran), respectively. Low CIA (65–57) and high ICV (1.64–1.05) values for the Louziba sandstones are suggestive of immature sediments that experienced mild to weak chemical weathering in their source areas. In contrast, early Paleozoic sandstones have higher CIA (80–69) and lower ICV (0.95–0.84) values, indicative of mature sediments with intensive chemical weathering. Despite the different degrees of chemical weathering, all sedimentary samples show positive correlation between TiO2 and Al2O3, and negative correlation in plot of (La/Yb)n vs Zr but lack any correlation between (La/Yb)n and P2O5. These data demonstrate sorting of resistant minerals, such as rutile, monazite and zircon, did not affect the chemical signatures of sampled detritus. Consequently, geochemical features, in combination with detrital age spectra, indicate that the Louziba Group and overlying sequences record sedimentation in collisional and passive settings, with the detritus being derived dominantly from intermediate to felsic igneous rocks.Late Mesoproterozoic (ca.1300–1000Ma) detritus in the lower Louziba Group were probably derived primarily from the Windmill Islands-Bunger Hills (1300–1050Ma) and Albany-Fraser Belt-Musgrave (1300–1050Ma) regions in East Antarctica-western Australia. Increasing input of early Neoproterozoic (∼990–950Ma) detritus in the uppermost Louziba Group indicates a greater contribution from the Eastern Ghats (ca. 990–950Ma) in eastern India and North Prince Charles Mountains (ca. 990–950Ma) in East Antarctica. This source change was synchronous with the formation of the Kuunga (560–530Ma) and Pinjarra (560–520Ma) Orogens during assembly of Gondwana. The sediments of the Huanglian Formation of the uppermost Louziba Group likely recorded this event. Overlying Paleozoic sequences then were deposited on passive margin with detritus being derived dominantly from an early Neoproterozoic source, e.g., eastern India and East Antarctica.

Geochemistry of upland lacustrine sediments from Serra dos Carajás, Southeastern Amazon, Brazil: Implications for catchment weathering, provenance, and sedimentary processes

A multi-proxy geochemical study of surficial sediments of an upland lake (Amendoim Lake), located in the Serra dos Carajás region, Brazil, was carried out to understand catchment weathering and provenance of sediments in the basin, as well as sedimentary processes. The carbon and nitrogen isotopic compositions of organic matter in the sediment were quite homogeneous, and suggest that the organic sources for this lake are mainly composed of palms and submerged macrophytes. The R-mode factor analysis indicates that most of the trace elements, including rare earth elements (REEs), are clustered with Al (Group 1), Si is grouped with Zr and Hf (Group 4), total organic carbon (TOC) with TS and Hg (Group 2), and Fe with Mn and As (Group 3). The elements of the Group 1 show strong positive correlations with Al, suggesting that these elements are hosted in detrital minerals during laterization in the catchment basin and not significantly affected by diagenesis. The high CIA values (96–99) of sediments together with their position in A-CN-K and log (Fe2O3/K2O) vs log (SiO2/Al2O3) plots indicate intense chemical weathering in source area. However, similar geochemical signatures between lake sediments and catchment lateritic crust indicates that mechanical erosion was the dominant sediment formation process. REE patterns normalized to chondrite along with geochemical indices (Al/K, Al/Ti, La/Th, La/Al, Ti/Zr, Zr/Hf, Th/Sc, Co/Th, Ba/Sr, and Eu/Eu*) also indicate that the sediments are mainly derived from laterite crust. This study provides reliable background information to reconstruct weathering processes and lake evolution in the Serra dos Carajás area.

Detrital mode and whole-rock geochemistry of the Miocene-Pliocene fluvial succession, Pishin Belt, Pakistan: implications on provenance and source area weathering in peripheral foreland basins

Detrital mode and geochemical composition of sandstones and mudstones of the Miocene Dasht Murgha Group (DMG) and Pliocene Malthanai Formation (MF) of the Pishin Belt, northwestern Pakistan, have been examined to identify their provenance and source area weathering. Sandstones of the Dasht Murgha Group and Malthanai Formation are lithic to sublitharenites, rich in quartz, and metamorphic and sedimentary lithic fragments, indicating a recycled orogenic source. LmLvLs plots show that the Dasht Murgha Group is rich in sedimentary and metamorphic lithic fragments (Lm35Lv18Ls47), while samples of the Malthanai Formation are overwhelmingly rich in sedimentary fragments (Lm14Lv10Ls76). Eocene Nisai Formation and Oligocene Khojak Formation within the Pishin Belt were mainly providing the sedimentary/metasedimentary detritus. High content of monocrystalline quartz (DMG 28.21 %; MF 30.7) and higher SiO2/Al2O3 ratios in sandstones (DMG 9.86; MF 11.98) also indicate high maturity of sandstones due to recycling of source terrain in collision orogens. High Cr/Ni (DMG 5.23; MF 6.17) and moderate Cr/V (DMG 3.96; MF 3.88) ratios suggest significant contributions from mafic and ultramafic detritus derived from Muslim Bagh-Zhob Ophiolite. Malthanai Formation has higher CIA and CIW values (68.96 and 77.53) than Dasht Murgha Group (63.87 and 70.93); however, they both indicate low to moderate weathering intensities. Dasht Murgha Group and Malthanai Formation have higher ICV values which indicate abundance of aluminous silicates, hence, showing moderate weathering of the source area. In A-CN-K diagrams, the samples make linear trend towards illite which suggests that the sediments were derived from a tectonically active source going through non-steady-state weathering, where the detritus has been derived from different zones of weathering profiles.

Petrography and geochemistry of modern river sediments in an equatorial environment (Rwenzori Mountains and Albertine rift, Uganda) — Implications for weathering and provenance

In hot–humid equatorial climate chemical weathering may be so strong that provenance signatures may be largely lost and even detritus derived from crystalline basement rocks reduced to quartzose sand. We tested this hypothesis in western Uganda, where stable plateau areas contrast with the active tectonic setting of the Albertine Rift (western branch of the East African Rift System, EARS), culminating in the strongly exhumed fault block of the >5000m high Rwenzori Mountains. In this setting, sediments derived from similar types of basement rocks including gneiss, schist, amphibolite, metasediments and granites can be traced from rapidly eroding high-altitude areas to low-altitude areas undergoing prolonged weathering. Sand and mud carried by 51 rivers overall in these two contrasting landscapes were sampled to study how and to what extent detrital modes are modified by the selective loss of unstable detrital minerals.Sediments generated in the high-relief Rwenzori Mountains show abundant feldspar (up to 32%) and rock fragments (up to 52%), which together with low SiO2/Al2O3 ratio and composition close to the Upper Continental Crust (UCC standard) reflect erosion in weathering-limited conditions. In the central Rwenzoris, low Th/Sc and Zr/Sc ratios, weak negative Eu anomaly, lower LaN/YbN values, and heavy-mineral assemblages with hornblende and epidote reflect the lithology of source rocks in the Buganda-Toro-Greenstone Belt. In contrast, sediments produced on the low-relief plateau have quartz content up to 98% and higher SiO2/Al2O3 ratio. Systematic loss of mobile elements is indicated by high chemical weathering indices CIA, PIA and WIP. However, provenance from metamorphic basement rocks is still indicated by heavy-mineral assemblages dominated by epidote and amphibole, whereas provenance from granitic rocks is revealed by high Th/Sc and Zr/Sc ratio, negative Eu anomaly and higher LaN/YbN values. We conclude that first-cycle sediments generated in high-relief areas preserve the original imprint of parent lithologies even in very humid equatorial climate. In low-relief areas, although weathering processes have proceeded over millions of years turning basement-derived detritus into an almost pure quartzose resistate, provenance signals are not erased entirely, and can be still retrieved from the residual heavy-mineral suite and relative abundance of high-field-strength trace elements.

Basalt and rhyo-dacite weathering and soil clay formation under subtropical climate in southern Brazil

The mineralogy of highly weathered soils is dominated by kaolinite-group clay minerals. However in the highlands of southern Brazil the subtropical climate (mean annual temperature ~15°C, mean annual precipitation 1900mm/yr) favored accumulation of organic matter and occurrence of 2:1 clay minerals. The objective of this work was to compare the weathering of the two main rock types in the volcanic plateau in the Rio Grande do Sul in southern Brazil in order to consider soil fertility and sustainability in this intensively cropped area. Two weathering profiles were sampled on basalt and rhyo-dacite at elevations ranging between 760 and 850ma.s.l. The soil minerals were analyzed by X-ray diffraction, chemical extractions, and FTIR spectroscopy. Weathering intensity was inferred from bulk chemical analyses and calculation of weathering indices (Chemical Index of Alteration, CIA; Weathering Intensity Scale, WIS) and elemental mass balance. Mineralogy was dominated by kaolinite and crystalline iron and aluminum oxy-hydroxides and 2:1 clay minerals. The surface horizons contained hydroxyl-Al-interlayered minerals originating from aluminization of smectite present in the saprolite under acidic conditions, with the important accumulation of organic matter. Results of mass balance calculation showed extensive loss of all elements (high Δ4Si, WIS and CIA) with residual accumulation of Fe and Al oxy-hydroxides. The degree of weathering is higher on rhyo-dacite compared to basalt although rhyo-dacite rock is richer in SiO2.Given the importance of 2:1 clay minerals on soil properties (e.g. sorption, aggregation) a better understanding of their formation and transformation according to geochemical conditions is a key issue in the management of these sub-tropical soils.

Late Miocene to early Pliocene climate variability off NW Africa (ODP Site 659)

A high resolution δ18O record obtained on benthic foraminifera (Cibicides wuellerstorfi) combined with clay mineralogy, major element concentrations and Sr–Nd isotopes have been investigated for the ODP Site 659 located off North Africa in order to reconstruct African monsoon intensity between 6.2 and 4.9Myr. Mineralogical and geochemical results indicate that sedimentation on the Cape Verde plateau results from a mixture of sediments deriving from the neighbouring Saharan arid region and the wet African equatorial zone (Sahelian Band), characterised by a low and a high state of weathering, respectively. Past variations in terrigenous inputs from these areas allow us to track past extension and displacement of the West African Monsoon–ITCZ system. Maximal summer insolation induced a northward migration of the inter-tropical convergence zone (ITCZ), wetter conditions in the Sahel, and a diminishing of wind erosion over the South Sahara and Sahel. In addition, there was an increase in river input to the Cape Verde plateau of sediments derived from southern sedimentary sources, which are characterised by high kaolinite contents and CIA values and low K/Al and Ti/Al ratios. On the other hand, minimal summer insolation induced a southward movement of the ITCZ, drier climate in the Sahel and stronger easterly winds resulting in an increase in dust transport from the Sahara to the Cape Verde Plateau. Dust particles are characterised by high illite and smectite content and by a low chemical state of weathering (low CIA values and high K/Al and Ti/Al ratios). Finally, our results provide new clues regarding the re-flooding of the Mediterranean Sea at the end of the MSC (5.33Myr). This event was associated with enhanced aridity in the Sahara, implying a reorganisation of the atmospheric circulation and a southward migration of the ITCZ.

Geochemistry of Mesozoic and Cenozoic sediments in the northern Qaidam basin, northeastern Tibetan Plateau: Implications for provenance and weathering

Whole-rock geochemical data of the Mesozoic and Cenozoic sediments in the northern Qaidam basin were used to reconstruct the provenance and chemical weathering history. Based on the fairly uniform REE patterns and trace element ratios, both the Mesozoic (LaCN/YbCN=10.37±1.75; Eu/Eu*=0.68±0.07; Th/Sc=1.11±0.38) and Cenozoic (LaCN/YbCN=9.77±0.62; Eu/Eu*=0.69±0.03; Th/Sc=0.97±0.14) mudstones were derived from a similar source area with acidic-intermediate rocks as dominant contributors, and two modeled mixtures composed of 60% granite, 35% quartzdiorite and 5% mafic rocks, and 55% granite, 40% quartzdiorite and 5% mafic rocks can be deduced as the potential source compositions for the Mesozoic and Cenozoic sediments, respectively. This conclusion is reinforced by the mudstone major element composition and sandstone petrography. The Early–Middle Jurassic mudstones have relatively high K-corrected CIA (84–93) and PIA (88–99) values, indicating intense chemical weathering conditions; while the Late Jurassic to Cenozoic sediments have variable corrected CIA (51–85) and PIA (50–92) values, implying predominantly mild to moderate weathering conditions. The chemical weathering history suggests that a warm and humid climate prevailed during the Early–Middle Jurassic, whereas a cool and semiarid to arid climate prevailed from the Late Jurassic up to now. This dramatic transition of climate may be attributed to multiple likely factors, including the restructuring of global atmospheric circulation patterns in response to the breakup of Pangea since Late Jurassic, and the orogenic rejuvenation and high elevation of the Qilian Mountains, northeastern Tibetan Plateau. Furthermore, the decreasing chemical weathering intensity over time indicates the increase of aridification and cooling of the climate culminating in the Late Cenozoic.

Collagen Type II and a Thermo-Responsive Polymer of N-Isopropylacrylamide Induce Arthritis Independent of Toll-Like Receptors : A Strong Influence by Major Histocompatibility Complex Class II and Ncf1 Genes

We established and characterized an arthritis mouse model using collagen type II (CII) and a thermo-responsive polymer, poly(N-isopropylacrylamide) (PNiPAAm). The new PNiPAAm adjuvant is TLR-independent, as all immunized TLR including MyD88-deficient mice developed an anti-CII response. Unlike other adjuvants, PNiPPAm did not skew the cytokine response (IL-1β, IFN-γ, IL-4, and IL-17), as there was no immune deviation towards any one type of immune spectrum after immunization with CII/PNiPPAm. Hence, using PNiPAAm, we studied the actual immune response to the self-protein, CII. We observed arthritis and autoimmunity development in several murine strains having different major histocompatibility complex (MHC) haplotypes after CII/PNiPAAm immunization but with a clear MHC association pattern. Interestingly, C57Bl/6 mice did not develop CII-induced arthritis, with PNiPAAm demonstrating absolute requirement for a classical adjuvant. Presence of a gene (Ncf1) mutation in the NADPH oxidation complex has a profound influence in arthritis and using PNiPAAm we could show that the high CIA severity in Ncf1 mutated mice is independent of any classical adjuvant. Macrophages, neutrophils, eosinophils, and osteoclasts but not mast cells dominated the inflamed joints. Furthermore, arthritis induction in the adjuvant-free, eosinophil-dependent Vβ12 DBA/1 mice could be shown to develop arthritis independent of eosinophils using CII/PNiPAAm. Thus, biocompatible and biodegradable PNiPAAm offers unique opportunities to study actual autoimmunity independent of TLR and a particular cytokine phenotype profile.

Combined statistical and petrological analysis of provenance and diagenetic history of mudrocks: Application to Alpine Tethydes shales (Sicily, Italy)

Mineralogical (X-ray diffraction) and geochemical (X-ray fluorescence) compositional data of Alpine Tethyde mudrocks from Sicily were used to unravel the interplay of provenance and diagenesis by means of multivariate statistical methods (PCA, discriminant function analysis) designed for non-negative constant-sum data. The Sicilian Alpine Tethydes comprise Cretaceous–Eocene scaly shales (Mt. Soro and Upper Scagliose Shales Units), Eocene–Oligocene variegated shales (Troina and Nicosia Units), and Miocene Numidian shales (Nicosia Unit). The use of biplots of clr-transformed variables allows discrimination of two different groups of shales. High CIA values reveal a strongly weathered source area especially for the Numidian shales. The analyses of mixed-layered clay minerals indicate that the behaviour of K2O and geochemical affine elements is controlled by the diagenetic grade, which increases from south-west to north-east in response to tectonic loading. Ratios of selected elements (Zr, Y, La, Nb, Ti, Co, Cr, Ni), considered essentially unchanged with respect to the source rocks, are similar in the scaly and variegated shales, and resemble the Variscan Peloritani metamorphic basement. In contrast, the distinct geochemical signature of the Numidian shales reveals a source similar to the average cratonic sandstone, suggesting derivation from external sources located in the foreland. Multivariate discriminant analysis performed on selected trace elements confirms the previous suggestions.

Heavy Minerals and Rare Earth Elements in Coastal and Inland Dune Sands of El Vizcaino Desert, Baja California Peninsula, Mexico

A heavy mineral (HM) study and light and heavy rare earth elements (LREE and HREE) analysis were performed in coastal and inland dune sands, El Vizcaino Desert, Central Baja California Peninsula, Mexico. Our study shows high abundances of hornblende and apatite in the El Vizcaino dune sands, suggesting a dominance of granodiorites/intermediate plutonic rocks and marine authigenic phosphorite in the dune sands. There is a relationship between unstable heavy minerals like hornblende, pyroxenes, and sphene, and heavy rare earth elements (HREE) that suggests that unstable heavy minerals are potential carriers of HREE in the dune fields. However, there is a slight depletion of HREE in relation to LREE, especially in one locality of the inland dunes probably associated with the wind regime and weathering of unstable heavy minerals in the sands. Inland, transitional, and coastal dune fields can be observed as different dune provinces by means of grouping HM and REE data in two separate dendograms. It seems that HREE are correlated with fine-grained sand sizes and correlated with high CIA values linked to slightly weathered sands.

Sedimentation in the Venezuelan Basin, Circulation in the Caribbean Sea, and Onset of Northern Hemisphere Glaciation

Currents of the equatorial Atlantic and Tethys Sea migrated westward through the Caribbean Sea and Panama seaway from Cretaceous to Oligocene time. At about 25 Ma currents reversed direction, allowing cool Pacific waters (Peruvian current) to enter the Caribbean. Almost coincident with the reversal was a dramatic increase in Central American explosive volcanism, which resulted in development of large volcanic centers, and an abundance of suspended volcanic detritus in Pacific and southern Caribbean waters. The detritus was transported by Pacific currents across the Caribbean to be deposited as pelagic mud in the Venezuelan Basin during 25 to 15 Ma. These cool, turbid Pacific waters led to a dramatic reduction in the diversity of hermatypic corals. At about 15 Ma Venezuelan Basin pelagic sediments record a dramatic change in provenance, with derivation of highly weathered detritus from the South American continent, probably the result of renewed uplift of the northern Andes. Exceptionally abundant kaolinite, reflected in high CIA values of the pelagic mud, indicates that materials of the Andean hinterlands suffered intense chemical weathering from at least 15 Ma to the present, with the most intense weathering occurring during Pliocene time. Weathering was somewhat less intense during the cool Pleistocene Period. These findings bear on Northern Hemisphere glaciation. With the Panama Seaway open, cool Pacific waters entered the Caribbean between 25 and 12 Ma, and probably mixed with warm equatorial Atlantic waters to form the Gulf Stream. Closure of the seaway commenced about 12 Ma and finally closed about 3.5 Ma. This resulted in a warmer Gulf Stream, derived entirely from equatorial Atlantic waters. Dramatically increased pelagic sedimentation rates in the Venezuelan Basin since 4 Ma suggest intensification of the Atlantic equatorial current and strengthening of equatorial Atlantic summer storms (hurricanes), both of which strengthened the Gulf Stream current. Resultant northward heat transport by a warm, intensified Gulf Stream led to increased precipitation and promoted glaciation in the Northern Hemisphere.

Geochemical and Nd/Pb Isotopic Evidence for the Provenance of the Early Proterozoic Virginia Formation, Minnesota. Implications for the Tectonic Setting of the Animikie Basin

The Early Proterozoic Animikie Basin is located at a major crustal boundary between Archean crust of the Superior craton and penecontemporaneous Early Proterozoic crust of the Wisconsin magmatic terranes. The provenance of sedimentary rocks within the Animikie Basin constrains the tectonic evolution of the Early Proterozoic margin of the Superior craton. Published geological, petrographic, and Nd-Pb isotopic data indicate that the Pokegama Quartzite (lower passive margin unit of the Animikie Group) has a Late Archean provenance. In contrast, the Virginia Formation (upper deep water turbidite-shale unit of the Animikie Group) was derived from dominantly Early Proterozoic sources, with isotopic compositions similar to igneous rocks of the Wisconsin magmatic terranes. Shales of the Virginia Formation have Nd depleted mantle model ages 2.14 to 2.35 Ga, and of 7.81 to 8.09. Ashes within the Virginia are more evolved geochemically than shales and have younger model ages of 1.86 and 1.99 Ga, and similar Shales of the Virginia Formation have high abundances and light rare earth element-enriched chondrite-normalized patterns, large and variable Eu anomalies, high CIA, high La/Th and low Th/Sc and Th/U. The provenance of the Virginia Formation can best be described as young differentiated arc. These data, combined with other data presented here and from published research and combined with published structural and stratigraphic constraints, are used to propose a model for the tectonic evolution of the Superior continental margin, beginning as a passive margin within a marginal back arc basin, and ending as a telescoped back arc basin that is inferred to have closed as a result of some change in relative plate convergence.