Petrogenesis of fertile mantle peridotites from the Monte del Estado massif (Southwest Puerto Rico): a preserved section of Proto-Caribbean lithospheric mantle?

The Monte del Estado massif is the largest and northernmost serpentinized peridotite belt in southwest Puerto Rico. It is mainly composed of spinel lherzolite and minor harzburgite with variable clinopyroxene modal abundances. Mineral and whole rock major and trace element compositions of peridotites coincide with those of fertile abyssal mantle rocks from mid ocean ridges. Peridotites lost 2-14 wt% of relative MgO and variable amounts of CaO by serpentinization and seafloor weathering. HREE contents in whole rock indicate that the Monte del Estado peridotites are residues after low to moderate degrees (2-15%) of fractional partial melting in the spinel stability field. However, very low LREE/HREE and MREE/HREE in clinopyroxene cannot be explained by melting models of a spinel lherzolite source and support that the Monte del Estado peridotites experienced initial low fractional melting degrees (~ 4%) in the garnet stability field. The relative enrichment of LREE in whole rock is not due to alteration processes but probably reflects the capture of percolating fluid/melt fractions or the crystallization of sub-percent amounts of hydrous minerals (e.g., amphibole, phlogopite) along grain boundaries or as microinclusions in minerals. We propose that the Monte del Estado peridotite belt represents a section of ancient Proto-Caribbean (Atlantic) lithospheric mantle originated by seafloor spreading between North and South America in the Late Jurassic- Early Cretaceous. This portion of oceanic lithospheric mantle was subsequently trapped in the forearc region of the Greater Antilles paleo-island arc generated by the northward subduction of the Caribbean plate beneath the Proto-Caribbean ocean. Finally, the Monte del Estado peridotites belt was emplaced in the Early Cretaceous probably as result of the change in subduction polarity of the Greater Antilles paleo-island arc without having been significantly modified by subduction processe

The system diopside-nepheline-leucite

The system diopside-nepheline-leucite, representing a join in the undersaturated part of the system nepheline (Ne)-kalsilite (Ks)-CaO-MgO-SiO2, has been investigated at atmospheric pressure. The system is pseudoternary and cuts the primary phase volumes of forsterite solid solution (Foss), diopside solid solution (Diss), nepheline solid solution (Ness), carnegieite solid solution (Cgss), and leucite solid solution (Lcss). Melilite (Mel) occurs as a subliquidus phase. The phase diagram has two four-phase points: 1. one at 1275 ± 5°C and Di60Ne8Lc32 where liquid coexists with Foss, Diss and Lcss, corresponding to olivine (Ol) leucitite; 2. the other at 1194 ± 5°C and Di27.5Ne29.5Lc43 where Ness, Foss and Lcss coexist with liquid, corresponding to Ol-Ne italite. With decreasing temperature, liquid moves from point (1) to a five-phase assemblage (3) where liquid is in equilibrium with Foss, Diss, Mel and Lcss (1258 ± 5°C), which is representative of Ol-Mel-leucitite. From point (2) liquid moves to a second five-phase assemblage (4), where Foss, Mel, Ness, Lcss and liquid are in equilibrium (1175 ± 5°C, corresponding to a Lc-Ne katungite. The assemblage Foss+Ness+Diss+Mel+Lcss+ liquid, is reached between 1168° and 1100° C and corresponds to Ol-Mel-Ne leucitite. Foss reacts with liquid and disappears. Near the point (1) it disappears at 1135 ± 10°C, whereas near the point (2) it reacts out at 1060 ± 10°C. Near the join Di-Ne it disappears at 950 ± 10°C. The final assemblage in the system is representative of Mel-Ne leucitite

Petrogenesis of fertile mantle peridotites from th Monte del Estado massif (Southwest Puerto Rico): a preserved section of Proto-Caribbean lithospheric mantle?

The Monte del Estado massif is the largest and northernmost serpentinized peridotite belt in southwest Puerto Rico. It is mainly composed of spinel lherzolite and minor harzburgite with variable clinopyroxene modal abundances. Mineral and whole rock major and trace element compositions of peridotites coincide with those of fertile abyssal mantle rocks from mid ocean ridges. Peridotites lost 2-14 wt% of relative MgO and variable amounts of CaO by serpentinization and seafloor weathering. HREE contents in whole rock indicate that the Monte del Estado peridotites are residues after low to moderate degrees (2-15%) of fractional partial melting in the spinel stability field. However, very low LREE/HREE and MREE/HREE in clinopyroxene cannot be explained by melting models of a spinel lherzolite source and support that the Monte del Estado peridotites experienced initial low fractional melting degrees (~ 4%) in the garnet stability field. The relative enrichment of LREE in whole rock is not due to alteration processes but probably reflects the capture of percolating fluid/melt fractions or the crystallization of sub-percent amounts of hydrous minerals (e.g., amphibole, phlogopite) along grain boundaries or as microinclusions in minerals. We propose that the Monte del Estado peridotite belt represents a section of ancient Proto-Caribbean (Atlantic) lithospheric mantle originated by seafloor spreading between North and South America in the Late Jurassic- Early Cretaceous. This portion of oceanic lithospheric mantle was subsequently trapped in the forearc region of the Greater Antilles paleo-island arc generated by the northward subduction of the Caribbean plate beneath the Proto-Caribbean ocean. Finally, the Monte del Estado peridotites belt was emplaced in the Early Cretaceous probably as result of the change in subduction polarity of the Greater Antilles paleo-island arc without having been significantly modified by subduction processes