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CINTRON FRANQUI, NADJA OMARA,CHOI, SUNG HI,LEE, DER-CHUEN GeoScienceWorld 2017 Geological magazine Vol.154 No.1
<P>The geology of Puerto Rico is divided into three regions: the north, central and SW igneous provinces. Characterized by its Jurassic ophiolitic melange basement, lithology of the SW Igneous Province (SIP) is not related to either of the other two provinces. The ophiolitic melange is exposed in three peridotite belts: Monte del Estado, Rio Guanajibo and Sierra Bermeja. We present geochemical data to identify the tectonic setting of the SIP peridotite formation and its relation to the evolution of the Caribbean Plate. Comparisons of spinel Cr no. (13-21), Mg no. (63.3-69.6) and TiO2 suggest an abyssal peridotite origin; however, only Sierra Bermeja presents high TiO2 characteristics of a mid-ocean-ridge-basalt- (MORB-) like melt reaction. Temperatures determined with two-pyroxene geothermometers indicated a cold thermal regime of c. 800-1050 degrees C, with characteristics of large-offset transform fault abyssal peridotites. The geochemistry and Sr-Nd-Hf-Pb isotopic compositions of basalts within the melange were also analysed. Las Palmas amphibolites exhibited normal-MORB-like rare earth element (REE) and trace-element patterns, whereas metabasalts and Lower Cajul basalts exhibited island-arc tholeiitic-like patterns. Highly radiogenic Sr isotopes (0.70339-0.70562) of the basalts suggest seawater alteration; however, Pb-Pb and Nd-Hf isotope correlations represent the primary compositions of a Pacific/Atlantic MORB source for the amphibolites, metabasalts and Lower Cajul basalts. We propose that the SIP ophiolitic melange was formed along a large-offset transform fault, which initiated subduction and preserved both proto-Pacific and proto-Caribbean lithospheric mantle. Younger Upper Cajul basalts exhibited enriched-MORB-like geochemical and isotopic signatures, which can be attributed to a tectonized Caribbean ocean plateau.</P>
Jung, Seung Gi,Choi, Sung Hi,Ji, Kang Hyeun,Ryu, Jong-Sik,Lee, Der-Chuen Elsevier 2019 Lithos Vol.350 No.-
<P><B>Abstract</B></P> <P>Oldoinyo Lengai is the only active carbonatite volcano within the East African Rift Valley in northern Tanzania. The volcano is dominated by peralkaline silicate rocks with natrocarbonatites. This study presents new mineralogical and geochemical data, including Sr–Nd–Pb–Hf–Mg isotopic compositions, for volcanic rocks at Oldoinyo Lengai and lavas from the nearby Gregory Rift Valley. The samples analyzed in this study include olivine melilitite, melanephelinite, wollastonite nephelinite, and phonolite. The olivine melilitites and melanephelinites have highly fractionated rare-earth-element (REE) patterns with (La/Yb)<SUB>N</SUB> values of 26.4–64.9, suggesting that they formed from magmas generated by low-degree (up to ∼7%) of partial melting within the garnet stability field. The wollastonite nephelinites have much higher (La/Sm)<SUB>N</SUB> values but lower (Sm/Yb)<SUB>N</SUB> values relative to typical oceanic island basalts (OIB), with flat heavy REE patterns [(La/Yb)<SUB>N</SUB> = ∼22]. The phonolites have elevated REE abundances but with patterns intermediate between the other two sample groups [(La/Yb)<SUB>N</SUB> = ∼41]. All samples have primitive-mantle-normalized incompatible element patterns that are characterized by negative K and Rb anomalies but no significant Eu anomalies. They also have elevated Yb contents relative to the compositions of modeled garnet peridotite-derived melts, suggesting that they were derived from a sublithospheric source containing enriched HIMU-like recycled oceanic crustal material. However, the wollastonite nephelinites have significantly positive Ba, U, Sr, and Pb anomalies similar to those found within the Oldoinyo Lengai natrocarbonatites. The wollastonite nephelinites might have been sourced from a region of sub-continental lithospheric mantle (SCLM) that was previously metasomatized by interaction with carbonatite melts. The phonolites in the study area have also weakly positive Pb and Sr anomalies indicative of some interaction with the SCLM. All samples have δ<SUP>26</SUP>Mg values (−0.39‰ ± 0.07‰) lighter than the composition of normal mantle material (−0.25‰ ± 0.04‰). In addition, a negative correlation between δ<SUP>26</SUP>Mg values and MgO concentrations suggests derivation from a source region containing recycled carbonate. The samples from the study area define a mixing array between HIMU- and EM1-type OIB in Sr–Nd and Pb–Pb isotopic correlation diagrams, and have pronounced Nd–Hf isotopic decoupling, plotting below the mantle regression line in Nd–Hf isotopic space. The negative deviation from the Nd–Hf isotopic mantle array and the presence of an EM1-type mantle component in the Sr–Nd isotopic compositions of the Oldoinyo Lengai volcanic rocks can be generated by recycling of E-MORB-type oceanic crustal material with an age of 1.5–1.0 Ga.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Oldoinyo Lengai lavas have Sr-Nd-Pb isotopic systematics defining a HIMU-EM1 array. </LI> <LI> They show Nd–Hf isotopic decoupling, plotting below the Nd–Hf mantle array. </LI> <LI> The mantle source is composed of garnet peridotite and recycled E-MORB-type slab. </LI> <LI> Their light Mg isotopic compositions are likely inherited from carbonated SCLM. </LI> </UL> </P>