Integrated provenance analysis of a convergent retroarc foreland system: U–Pb ages,heavy minerals,Nd isotopes,and sandstone compositions of the Middle Magdalena Valley basin,northern Andes,Colombia

Sediment provenance analysis remains a powerful method for testing hypotheses on the temporal and spatial evolution of uplifted source regions, but issues such as recycling, nonunique sources, and pre- and post-depositional modifications may complicate interpretation of results from individual provenance techniques. Convergent retroarc systems commonly contain sediment sources that are sufficiently diverse (continental magmatic arc, fold–thrust belt, and stable craton) to enable explicit provenance assessments. In this paper, we combine detrital zircon U–Pb geochronology, heavy mineral identification, Nd isotopic analyses, conventional sandstone petrography, and paleocurrent measurements to reconstruct the clastic provenance history of a long-lived sedimentary basin now exposed in an intermontane zone of the northern Andean hinterland of Colombia. The Middle Magdalena Valley basin, situated between the Central Cordillera and Eastern Cordillera, contains a 5–10 km-thick succession of Upper Cretaceous to Quaternary fill. The integrated techniques show a pronounced change in provenance during the Paleocene transition from the lower to upper Lisama Formation. We interpret this as a shift from an eastern cratonic source to a western Andean source composed of magmatic-arc rocks uplifted during initial shortening of the Central Cordillera. The appearance of detrital chloritoid and a shift to more negative ε_Nd(t=0) values in middle Eocene strata of the middle La Paz Formation are attributed to shortening-related exhumation of a continental basement block (La Cira–Infantas paleohigh), now buried, along the axis of the Magdalena Valley. The diverse provenance proxies also show distinct changes during middle to late Eocene deposition of the Esmeraldas Formation that likely reflect initial rock uplift and exhumation of the fold–thrust belt defining the Eastern Cordillera. Upsection, detrital zircon U–Pb ages and heavy mineral assemblages for Oligocene and younger clastic deposits indicate that the Mesozoic sedimentary cover of the Eastern Cordillera was recycled during continued Cenozoic shortening. Our multidisciplinary provenance study refines the tectonic history of the Colombian Andes and demonstrates that uncertainties related to sediment recycling, nonunique sources, source heterogeneity, and climate in interpreting provenance data can be minimized via an integrated approach.     

Major element, REE, and Pb, Nd and Sr isotopic geochemistry of Cenozoic volcanic rocks of eastern China: implications for their origin from suboceanic-type mantle reservoirs

Major- and rare-earth-element (REE) concentrations and UThPb, SmNd, and RbSr isotope systematics are reported for Cenozoic volcanic rocks from northeastern and eastern China. These volcanic rocks, characteristically lacking the calc-alkaline suite of orogenic belts, were emplaced in a rift system which formed in response to the subduction of the western Pacific plate beneath the eastern Asiatic continental margin. The rocks sampled range from basanite and alkali olivine basalt, through olivine tholeiite and quartz tholeiite, to potassic basalts, alkali trachytes, pantellerite, and limburgite. These rock suites represent the volcanic centers of Datong, Hanobar, Kuandian, Changbaishan and Wudalianchi in northeastern China, and Mingxi in the Fujian Province of eastern China.The major-element and REE geochemistry is characteristic of each volcanic suite broadly evolving through cogenetic magmatic processes. Some of the outstanding features of the isotopic correlation arrays are as follows: (1) NdSr shows an anticorrelation within the field of ocean island basalts, extending from the MORB end-member to an enriched, time-averaged high Rb/Sr and Nd/Sr end-member (EM1), (2) SrPb also shows an anticorrelation, similar to that of Hawaiian and walvis Ridge basalts, (3) NdPb shows a positive correlation, and (4) the ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb plot shows linear arrays parallel to the general trend (NHRL) for MORB on both sides of the geochron, although in the ²⁰⁸Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb plot the linear array is significantly displaced above the NHRL in a pattern similar to that of the oceanic island basalts that show the Dupal signatures. In all isotope correlation patterns, the data arrays define two different mantle components—a MORB-like component and an enriched mantle component. The isotopic data presented here clearly demonstrate the existence of Dupal compositions in the sources of the continental volcanic rocks of eastern China. We suggest that the subcontinental mantle beneath eastern China served as the reservoir for the EMI component, and that the MORB component was either introduced by subduction of the Kula-Pacific Ridge beneath the Asiatic plate in the Late Cretaceous, as proposed by Uyeda and Miyashiro, or by upwellings in the subcontinental asthenosphere due to subduction.     

U-Pb age of the Diana Complex and Adirondack granulite petrogenesis

U-Pb isotopic analyses of eight single and multi-grain zircon fractions separated from a syenite of the Diana Complex of the Adirondack Mountains do not define a single linear array, but a scatter along a chord that intersects the Concordia curve at 1145 ± 29 and 285 ± 204 Ma. For the most concordant analyses, the²⁰⁷Pb/²⁰⁶Pb ages range between 1115 and 1150 Ma. Detailed petrographic studies revealed that most grains contained at least two phases of zircon growth, either primary magmatic cores enclosed by variable thickness of metamorphic overgrowths or magmatic portions enclosing presumably older xenocrystic zircon cores. The magmatic portions are characterized by typical dipyramidal prismatic zoning and numerous black inclusions that make them quite distinct from adjacent overgrowths or cores when observed in polarizing light microscopy and in backscattered electron micrographs. Careful handpicking and analysis of the “best” magmatic grains, devoid of visible overgrowth of core material, produced two nearly concordant points that along with two of the multi-grain analyses yielded an upper-intercept age of 1118 ± 2.8 Ma and a lowerintercept age of 251 ± 13 Ma. The older age is interpreted as the crystallization age of the syenite and the younger one is consistent with late stage uplift of the Appalachian region. The 1118 Ma age for the Diana Complex, some 35 Ma younger than previously believed, is now approximately synchronous with the main Adirondack anorthosite intrusion, implying a cogenetic relationship among the various meta-igneous rocks of the Adirondacks. The retention of a high-temperature contact metamorphic aureole around Diana convincingly places the timing of Adirondack regional metamorphism as early as 1118 Ma. This result also implies that the sources of anomalous hightemperature during granulite metamorphism are the syn-metamorphic intrusions, such as the Diana Complex.     

Rare gases,water, and carbon in kaersutites

Kaersutites from Kakanui, New Zealand and from three localities in the southwestern United States have been analyzed for rare gases, water and carbon to investigate the volatile signature of the sub-continental mantle. This study does not confirm the high ³He/⁴He and ²¹Ne/²²Ne ratios reported by Saito et al. [1] for the Kakanui kaersutite. Instead, a ³He/⁴He ratio of 6 R_A and atmospheric ²¹Ne/²²Ne ratios were measured which are consistent with our current knowledge of the earth's mantle. A low ⁴⁰Ar/³⁶Ar of 320 and more than 10^?8 cm³/g of ³⁶Ar confirms the argon results of Saito et al. and indicates that significant quantities of ³⁶Ar reside in this portion of the mantle. Kaersutites from the southwestern United States (Arizona) have a heterogeneous helium isotope signature, ranging from 8.8 R_A at San Carlos to 0.46 at Hoover Dam. All D/H ratios for the water in kaersutites (?56‰ to ?78‰) represent typical mantle values with no apparent correlation with ³He/⁴He. The correlation of increasing carbon content (140–400 ppm) with increasing δ¹³C (?24.5‰ to ?16.7‰) may reflect differences in the proportions of oxidized and reduced carbon in these samples.     

Sm-Nd,K-Ar and petrologic study of some kimberlites from eastern United States and their implication for mantle evolution

We provide new data on Sm-Nd systematics, K-Ar dating and the major element chemistry of kimberlites from the eastern United States (mostly from central New York State) and their constituent mineral phases of olivine, clinopyroxene, garnet, phlogopite and perovskite. In addition, we report Nd-isotopes in a few kimberlites from South Africa, Lesotho and from the eastern part of China. The major element compositions of the New York dike rocks and of their constituent minerals including a xenolith of eclogite are comparable with those from the Kimberley area in South Africa. The K-Ar age of emplacement of the New York dikes is further established to be 143 Ma.We have analyzed the Nd-isotopic composition of the following kimberlites and related rocks: Nine kimberlite pipes from South Africa and Lesotho, two from southern India; one from the U.S.S.R., fifteen kimberlite pipes and related dike rocks from eastern and central U.S. and two pipes from the Shandong Province of eastern China. The age of emplacement of these kimberlites ranges from 1300 million years to 90 million years. The initial Nd-isotopic compositions of these kimberlitic rocks expressed as _Nd ^I with respect to a chondritic bulk-earth growth-curve show a range between 0 and +4, with the majority of the kimberlites being in the range 0 to +2. This range is not matched by any other suite of mantle-derived igneous rocks. This result strengthens our earlier conclusion that kimberlitic liquids are derived from a relatively primeval and unique mantle reservoir with a nearly chondritic Sm/Nd ratio.     

Sm-Nd Isotopic and Geochemical Study of the Archean Tonalite-Amphibolite association from the eastern Indian Craton

We report the results of a Sm-Nd isotopic, major element and rare earth element (REE) study of the Older Metamorphic Group (OMG) tonalite-amphibolite association of the eastern Indian Craton. The Older Metamorphic Tonalite Gneisses (OMTG) have been previously dated to be 3.8 Ga using Sm-Nd isotope systematies, and 3.2–3.4 Ga by Rb-Sr and Pb-Pb dating. The results of this study indicate that the protoliths of the OMG amphibolites are 3.3 Ga isochron age=3.30±0.06 Ga, _Nd= +0.9 ± 0.7), and therefore, the OMTG, which intrude into the associated amphibolites, cannot be any older than 3.3 Ga. The amphibolites display light REE enrichment ((Ce/Yb)_N=2.2–6.7; La=30–100 x chondrite) and nearly flat heavy REE patterns ((Tb/Lu)_N=1.2–1.9); the basaltic parents of the amphibolites were probably generated by the partial melting of a spinel lherzolite mantle. Strong linear relationships between the amphibolites and tonalites in ¹⁴⁷Sm/¹⁴⁴Nd-¹⁴³Nd/¹⁴⁴Nd space (isochron age =3.29±0.04 Ga, _Nd= +0.8 ± 0.8) imply that they are genetically related. The tonalites display fractionated REE patterns (La=100–300 x chondrite) with moderate heavy REE depletions ((Tb/Lu)_N=1.9–3.4). The isotopic, major element and REE data are consistent with the derivation of the OMTG from partial melting of OMG amphibolites or equivalent rocks at amphibolegarnet stabilization depths. An initial _Nd(t) value of +0.9±0.7 for the amphibolites indicates the presence of a slightly depleted mantle source at 3.3 Ga with ¹⁴⁷Sm/¹⁴⁴Nd. between 0.20 and 0.22. It is suggested that the growth of continental crust in the eastern Indian craton occurred in response to magmatic underplating in a plume setting.     

Basalt-rhyolite volcanism by MORB-continental crust interaction: Nd,Sr-isotopic and geochemical evidence from southern San Joaquin Basin,California

The early miocene Tecuya volcanic center in the southern San Joaquin basin of California consists of flows and tuffs of basalt and rhyolite that erupted, closely spaced in time, in both submarine and subaerial conditions. The rhyolites are overlain by the basalts and constitute approximately 45% of a total of at least 180 km³ of the Tecuya volcanic rocks. The basalts have _Nd(t) values of +2 to +6 and (⁸⁷Sr/⁸⁶Sr)_i values between 0.7035 and 0.7052. These rocks show LREE enrichment [(La/Yb)_N =2.4–5.5; La=28–150 times chondrite] and higher Th/U, Th/Ta, Rb/Ta, Ba/Ta, Cs/Rb but lower K/Rb ratios than MORB. Combined major- and trace-element, and Sr–Nd isotopic data suggest the involvement of subcontinental lithosphere, depleted upper mantle source (MORB), and local continental crust in the basalt petrogenesis. _Nd(t) values in rhyolites vary from +1.5 to +3.7 while (⁸⁷Sr/⁸⁶Sr)_i ratios range from 0.7051 to 0.7064. The rhyolites display LREE enrichment [(La/Yb)_N=10; La=100 times chondrite] along with a distinct negative Eu anomaly (Eu/Eu^*=0.75) and depletion of Ti and P. Mixing relations in (⁸⁷/⁸⁶Sr)_i – _Nd(t) space among basalts, rhyolites, and local continental crust indicate that the Tecuya rhyolites were produced by assimilation of variable amounts of continental crust by MORB-related magmas and subcontinental lithosphere-derived melts. This conclusion is supported by the synchroneity of Tecuya volcanism at 22 Ma with interaction of a segment of the East Pacific Rise along the southern California margin. The Tecuya volcanic rocks thus provide an example for the generation of rhyolitic melts owing to crustal assimilation by basaltic melts during mid-oceanic ridge-induced magmatism along a continental margin.     

Trace elements and Sr-isotopes in some mantle-derived hydrous minerals and their significance

New analyses of K, Rb, Sr and Ba contents and the ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of eight amphiboles, one phlogopite, two diopsides and one host alkalic basalt for an amphibole are reported: The samples are mostly inclusions in alkalic basalts and occur in association with peridotite inclusions. Two of the samples are from alpine-type peridotite bodies — one from the Etang de Lhers massif in the French Pyrenees and the other from the Finero massif in the Ivrea zone in northern Italy. The kaersutites come from the following localities: Hoover Dam, Arizona; Deadman Lake, California; Massif Central, France; Queensland; Spring Mountain, New South Wales.The data indicate that kaersutitic amphiboles are genetically unrelated to their host basalts. The isotopic and trace element data of these amphiboles further strengthens the suggestion of BASU and MURTHY (1977) that kaersutites play a significant role in ocean ridge basalt genesis. In addition, pargasitic amphibole with higher ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios, if present, may be important in the source regions of alkalic basalts.The bulk amphibole lherzolite from Lherz has the $\text{K}\text{Rb}$ratio and ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio appropriate for source material of ridge tholeiites. If the diopside and the amphibole in this rock had isotopically equilibrated under upper mantle conditions, the data show the time of last equilibration to be approximately 735 m.y., in contrast to the young emplacement age of the ultramafic massif.The coexisting phlogopite and diopside in the spinel lherzolite inclusion from Kilbourne Hole, New Mexico, show, surprisingly, isotopic equilibration under upper mantle conditions despite their drastically different $\text{Rb}\text{Sr}$ ratios. The data show that the phlogopite must have formed very recently in the upper mantle. This phlogopite also has a high $\text{K}\text{Rb}$ ratio (1133), contrary to the commonly held view that mantle phlogopites have low $\text{K}\text{Rb}$ ratios. The coexisting diopside shows high K content (778 ppm) and a lower $\text{K}\text{Rb}$ ratio than the phlogopite. This phlogopite lherzolite has trace elemental and isotopic characteristics that may be adequate for the origin of alkalic basalts upon partial melting.