U. Mass Lowell |
Prof. Nelson Eby | Department of Environmental, Earth, & Atmospheric Sciences |
| White Mountain Petrographic Province |
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Petrological, geochemical (trace elements by INAA), geochronological, and isotopic investigations of the various magma series of the White Mountain petrographic province have been ongoing since 1982. A number of papers have been published on this province. |
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Eby, G. N. Alkaline rocks comprise a minor amount of the total volume of igneous rocks, but in terms of variety and complexity they have challenged petrologic thinking for decades. Alkaline magmatism is widely distributed both spatially and throughout geologic time. Most alkaline provinces show significant lithological diversity and classic examples of this diversity are the Chilwa Alkaline Province (CAP) of southern Malawi and the Monteregian Hills - White Mountain (MHWM) province of Quebec and New England. CAP magmatism started at ca 133 Ma and continued to ca 110 Ma. Initial magmatism was marked by the eruption of nephelinitic and basanitic magmas (now preserved as large enclaves in later syenite intrusions). Subsequent intrusions systematically progressed from silica undersaturated sodalite-nepheline syenites through syenites, and the igneous activity culminated with the emplacement of a large alkali granite body. Spatially related carbonatite magmatism occurred ca 126 Ma. Sr, Nd, and Pb isotopic data show that the magmas were all derived from a depleted mantle source with OIB-like characteristics. Both trace element and isotopic data support the contention that the trend towards silica oversaturation was largely due to greater amounts of crustal contamination, probably a reflection of both a longer residence time for the melts in the deep crust and an increase in crustal temperatures because of the earlier igneous activity. In contrast to the CAP, the bulk of the igneous activity in the MHWM occurred at ca 123 Ma and lithologic diversity is spatially distributed - carbonatites occur at the western end of the province and eastward the plutons become less silica undersaturated. Plutons emplaced into the folded Appalachian sequence are largely composed of silica saturated to silica oversaturated lithologies. There is a general increase in the volume of the plutons in an easterly direction. In terms of areal exposure felsic rocks are more abundant than mafic rocks, but geophysical data indicate that there are significant volumes of mafic rock at depth. With the exception of the carbonatites, throughout the province both mafic and felsic rocks show trace element distributions characteristic of OIBs. Both trace element and isotopic data support the derivation of the melts from an OIB-like source. Melting models suggest that in an easterly direction there is an increase in partial melting in the source region, and this is the most important parameter in determining the silica undersaturated or silica saturated characteristics of individual plutons. A secondary effect is interaction of the melts with crustal material. Eby, G. N. The Ossipee complex, a classic example of a ring-dike structure, is a member of the ca. 120 Ma Younger White Mountain magma series. An almost complete outer ring dike consists of porphyritic quartz syenite and subporphyritic granite. Porphyritic (feldspar phenocrysts) basalts and rhyolites are abundant within the ring structure. The basalts represent the only significant occurrence of mafic volcanics in the White Mountain Province. Alkali granite was emplaced as a sheet under the volcanic pile. Geophysical data indicate that mafic rocks are abundant in the conduit that fed the Ossipee structure. The basalts are divisible into three groups: nepheline-normative and high-Ti and low-Ti quartz-normative. Evolution of the basalts was controlled by plagioclase fractionation. The rhyolites are divisible into several geochemical groups, and differentiation within each group was controlled by feldspar fractionation. OIB normalized spider diagrams for the basalts are essentially flat, and slightly enriched relative to OIB, except for minor depletions in Sr and Ti and a significant enrichment in Cs (due to late stage hydrothermal alteration of the basalts). Rhyolite spider diagrams are similar in shape but show greater enrichment in most trace elements relative to the basalts, and significant depletion in Ba, Sr and Ti. Phase equilibria considerations indicate that the nepheline-normative basalts were erupted directly to the surface from deep levels in the crust (or upper mantle) while the quartz-normative basalts and rhyolites evolved at intermediate depths before their eruption. This difference in evolution is reflected in the isotopic systematics which show the influence of crustal contamination (87Sr/86Sr i = 0.704 to 0.721) in the evolution of the quartz-normative basalts and rhyolites. However, AFC modelling indicates that the total amount of crustal contamination is relatively small. Comparisons with other plutons in the Younger White Mountains and time correlative Monteregian Hills province of southern Quebec indicate that all these magmas were derived from the same source. The differences in magma composition (from silica-undersaturated to silica-saturated) is due to the degree of melting in the source region and the amount of crustal interaction. Mineral chemistry (Excel spreadsheet) Geology, Geochronology, and Geochemistry of the White Mountain Batholith, New Hampshire Eby, G. N., Krueger, H. L., and Creasy, J. W. The White Mountain batholith, central New Hampshire, is a member of the older White Mountain igneous province. The batholith consists of a number of overlapping centers of felsic magmatism comprised of quartz syenite porphyry ring dikes, alkali granite, metaluminous biotite granite, and felsic volcanics. Igneous activity occurred over an extended period of time from ca. 200 Ma to 155 Ma. Both chemically and mineralogically the various suites of the batholith are A-type granitoids. Absolute abundances of alkalis, total Fe, REE (except Eu), Y, Nb, Ta, Hf, Zr, Th, and U are high relative to other granitoid types. The mafic mineralogy is characterisitc of A-type granitoids: ferrohedenbergite, fayalite, ferrohastingsite, and riebeckite. Isotopic and trace-element data indicate that a number of the suites evolved by closed system crystal fractionation. The evolution of these suites was largely controlled by feldspar (perthite, K-feldspar, plagioclase), fayalite, ferrohedenbergite, and ilmenite fractionation. Locally contaminated phases attest to limited open system fractionation at high crustal levels. While Sr isotopic ratios are generally significantly greater than mantle values, trace-element ratios are typical of magmas derived from oceanic-island-basalt-like sources. Modeling shows that the high 87Sr/86Sr ratios can be explained by limited contamination of highly evolved magmas by radiogenic crust. The preferred model for the origin of the batholith involves the emplacement of mantle-derived melts into the base of the crust. These magmas evolve, with some crustal contamination, at this level. The resulting homogeneous magmas are then emplaced at higher levels in the crust where further crystal fractionation and contamination can occur. This is essentially the MASH model advanced by Hildreth and Moorbath (1988). Rock chemistry (Excel spreadsheet) The Monteregian Hills and White Mountain Alkaline Igneous Provinces, Eastern North America Eby, G. N. The Monteregian Hills and White Mountain provinces consist of stocks, plugs, ring-dyke complexes and several large granite bodies emplaced into Precambrian gneisses, flat-lying Cambro-Ordovician sediments and the deformed Lower Palaeozoic section of the Appalachian fold belt. Felsic rocks dominate in the Appalachian fold belt, while elsewhere mafic and ultramafic rocks are significant components of the plutons. Igneous activity extended from 240 to 90 Ma ago with two major periods of magmatism, correlated with events in the opening of the N Atlantic Ocean, occurring between 200 - 165 Ma and 140 - 110 Ma. Five major rock series have been identified: (1) undersaturated CO2-rich rocks, carbonatite and alnöite; (2) moderately to strongly undersaturated diorites - nepheline syenites; (3) slightly undersaturated to slightly oversaturated pyroxenites - gabbros - diorites - syenites; (4) alkali syenite - quartz syenite - granite; (5) metaluminous biotite granite. Series (1), (2) and (3) magmas were drawn from an isotopically depleted mantle which was enriched in incompatible elements shortly before or synchronous with melting. These magmas were produced by variable degrees of melting of garnet or spinel lherzolite. Series (4) and (5) magmas represent partial melts of a heterogeneous crustal section consisting of both meta-sedimentary and meta-igneous rocks of either Grenville (Precambrian) or Lower Palaeozoic age. Eby, G. N. The mafic alkaline dikes of the Monteregian Hills and younger White Mountain igneous provinces can be divided into three groups: (1) K2O-rich alnöites; (2) moderately to strongly undersaturated monchiquites, camptonites, and basanites; and (3) slightly undersaturated to critically saturated camptonites and alkali olivine basalts. The dikes were emplaced between 139 and 107 Ma, with the bulk of the activity occurring in three discrete intervals: 139 - 129, 121 - 117, and 110 - 107 Ma. The first two intervals correspond to the times of emplacement of the main Monteregian intrusions. There is no apparent geographic pattern to the ages. Chemical evolution of the group 2 and group 3 magmas was largely controlled by the removal of olivine, clinopyroxene, and Fe-Ti-rich oxides. The group 2 dikes are generally enriched in REE and have higher La/Yb ratios (18 - 28) than the group 3 dikes (La/Yb = 9 - 23). For the majority of the samples Zr/Hf ratios (30 - 43) and Rb/Ba x 102 ratios (4.8 - 11.6) fall in the range of primary basalts, but some samples have higher ratios, indicating crustal contamination. Trace-element models indicate that group 2 and group 3 magmas originated by variable degrees of melting of a metasomatized spinel lherzolite whereas the group 1 magmas most likely originated in a carbonated garnet lherzolite mantle. The thermal energy for the melting may have been provided by a mantle plume. Rock chemistry (Excel spreadsheet) Eby, G. N. The Monteregian Hills and younger White Mountain alkaline intrusions were emplaced into the Cambro-Ordovician sediments of the St. Lawrence Lowlands and the folded and thrusted Lower Paleozoic sequence of the Appalachian orogen. Age relations indicate that there is a fine-scale structure to the igneous activity, with slightly undersaturated to critically saturated rocks emplaced between 141 and 128 Ma and strongly undersaturated rocks emplaced between 121 and 117 Ma. Sr and Pb isotopic data for the mantle-derived alkali picrite, alkali olivine basalt and basanite magmas, indicate derivation from a depleted mantle similar to that which produces present-day oceanic island basalts. For the most isotopically primitive samples, decay corrected 87Sr/86Sr = 0.7030 - 0.7037, 206Pb/204Pb = 19.05 - 19.72, 207Pb/204Pb = 15.56 - 16.65, and 208Pb/204Pb = 38.64 - 39.26. On Pb-Sr isotope correlation diagrams the data define trends similar to those for MOR basalts, implying mantle heterogeneity which requires the presence of a component enriched in radiogenic Pb relative to Sr. The interaction of these isotopically primitive magmas with the crust can be defined in terms of a three component system: depleted mantle - Grenville age crust - Lower Paleozoic age crust. The granitic magmas were apparently derived from the Lower Paleozoic crust of the Appalachian orogen. For the mantle-derived magmas, Th/U ratios vary from 2.5 (estimated ratio for MORB source) to 5.1, with the mean value near that of the bulk earth. The variations in Th/U suggest mantle heterogeneity on a local scale, and the high Th/U of some samples suggests that the mantle was enriched in incompatible elements shortly before melting. The magmas derived by partial melting of the crust have Th/U of 3.3 to 8.7, and the higher ratios are associated with rocks crystallized from magmas that originated by melting of Lower Paleozoic sediments. The Sr and Pb isotopic data support the conclusion of Bell et al. (1982) that the subcontinental mantle under eastern Canada underwent a Precambrian depletion event. This depleted mantle apparently extends under the White Mountain province and is isotopically similar to the mantle that gives rise to oceanic island basalts. In contrast, Pb isotopic ratios for the New England Seamount chain (Tara and Hart, 1983), which apparently represents the oceanic extension of this magmatic activity, are significantly more radiogenic. It is possible that a mantle plume provided the heat energy, and perhaps metasomatic fluids, to trigger melting in the subcontinental mantle. whereas in the case of the oceanic extension the plume directly contributed to the observed magmatism, as reflected in the more radiogenic Pb ratios. Rock isotopic and chemical data (Excel spreadsheet) |
The
White Mountain petrographic province (WMPP) of New Hampshire, Vermont,
and Maine can be subdivided on a geochronological basis into an older
White Mountain series (emplaced ca 180 Ma) and a younger White
Mountain Series (emplaced ca 120 Ma, at the same time as the
various intrusions of the Monteregian Hills alkaline province).
Volumetrically the older White Mountain series is dominant and the White
Mountain batholith, the largest intrusive/extrusive unit, belongs to
this series. The major rock types are syenite, quartz syenite, and
metaluminous and peralkaline granites. Comendites are found in the White
Mountain batholith. Silica-undersaturated syenites occur at Red Hill and
Rattlesnake. Mafic rocks are rare. The younger White Mountain series
consists of small plutons many of which contain both mafic and felsic
plutonic rocks. Felsic volcanics are found at Merrymeeting Lake and
Ossipee, and basalts and andesites are found at Ossipee. With the
exception of Cuttingsville, in east-central Vermont, all lithologies are
silica-saturated to silica-oversaturated. Nepheline and/or sodalite-bearing
gabbros/diorites and syenites are found in volumetrically small amounts
at Cuttingsville.