Formation Hypotheses of White Mountain Magma Series
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- Julie Sophis
A grouping of igneous rocks, similar in chemical, texture, and mineralogical features which come from a common source magma and are within a similar time and space when intruded are considered a magma series (Lachance, 1978). The White Mountain Magma Series, WMMS, is located primarily in New Hampshire with a few related plutons mapped in Maine and Vermont. This series received much of its recognition in 1956 and earlier (McHone and Butler, 1984). The White Mountain Magma Series has been placed as part of the New England-Quebec province, representing igneous activity which is considered to be of similar ages and similar compositions that stretches over an area of 300km by 400km through southern Quebec and New England (McHone and Butler, 1984). This magmatism of the whole White Mountain igneous province is characterized as A-type (Eby, 1999). In looking specifically at the White Mountain Magma Series, two distinct time frames of igneous activity are found, one older and one younger.
The older igneous activity, referred to as the older White Mountain Igneous Province (OWM), dates to 220-155 Ma (Eby and Kennedy, 2004). Alkali syenites, quartz syenites, metaluminous granite, peralkaline granite, peralkaline rhyolites, and two areas of identified silica-undersaturated rock have been identified (Eby and Kennedy, 2004). There is an absence of mafic igneous rocks and this older area consists of multiple ring dikes (Eby and Kennedy, 2004).
The younger igneous activity, currently referred to as Monteregian Hills White Mountain Igneous Province (MHWM), is marked at 130-100 Ma (Eby and Kennedy, 2004). The majority of the magmatism is dated to have occurred in about 125 Ma; however, younger outliers exist (Armstrong and Stump, 1971; Foland and Faul, 1977; Eby and Kennedy, 2004). This younger activity consists of mainly of mafic alkaline suites and felsic rocks in the intrusions and of this series, small plugs and ring like structures are both present with the most evolved rocks being syenites and quarts with occurrences of biotite granite (Eby and Kennedy, 2004).
Many geologists have hypothesized the origin of the magma series. As advances in geological sciences have been made, along with advances in identification of rocks and dating, these hypotheses have evolved. Of these, one of the first major ideas include deep seated fractures in a northwest and east-west trending network that act as centers of low pressure and intrusions for melting (Chapman, 1968). A hypothesis of a hotspot origin has been supported by a greater range of geologists (Crough, 1981b; Duncan, 1984). A third major hypothesis to the origin of the WMMS involves rifting in line with the opening of the Atlantic (Foland and Faul, 1977; McHone, 1981; McHone and Butler, 1984). Since there is no decisive agreement on the origin of the White Mountain Magma Series, there have been advances in understanding the magma sources themselves (Eby et al., 1992).
The evolution of hypotheses surrounding the origin of the magmatism in the White Mountain Magma Series will be explored in this paper. The evolution of thought with incorporation of geological advances will be used to determine the current understanding of the White Mountain Magma Series.
Carleton Chapman was one of the first geologists to write about the formation of the WMMS. As published, it was postulated that there are two sets of deep seated fracture zones which form a lattice within the crust of the earth under the WMMS (Chapman, 1968). In this hypothesis, these zones had a lower pressure and underwent partial melting from which mafic magma intruded via rounded chambers and rose to the top of the crust (Chapman, 1968). The mapped absences of igneous activity were taken into account and justified to be due to inadequate melting in a particular region, prevention from overlying rock in allowing the magma to rise to the surface were it could be mapped, and that igneous rock could have been mistakenly missed in field work or covered by surface rock (Chapman, 1968). The lattice line structure proposed has little evidence to support it as there are no faults along the proposed structure of lines (McHone and Butler, 1984).
The hotspot model appears in a number of papers in which the WMMS is linked to a hotspot in with the North American plate moved over. An expanded version of the simple hotspot model has been made with the addition to support of the hotspot origin of the New England Seamount chain and the general movement of the North American plate over a hotspot (Crough, 1981b). In connecting the use of conodant, fission track, radiometric, and tectonic data, a hypothesis that this movement led to the regional uplift of New England was developed (Crough, 1981b). This uplift was at least 4km in comparison to the central Appalachian region (Crough, 1981b). Through the plotting of this data, the younger White Mountain Igneous Province forming via the Greater Meteor hotspot track is explained; however, the Older Igneous Province is not accounted for in this trace (Crough, 1981b). This argument has published faults; it is argued that due to lack of significant age progression there is a large data gap along the hotspot trace between the province and used kimberlite and seamounts (McHone, 1981). In addition to this gap, it is pointed out that although a portion of the data does fit the hotspot model, it excludes the Older Igneous Province, leaving many questions as to whether this is due to a mantle plume whose trace has been erased, later magmatism, or other events not known (McHone, 1981). .
In support of the hotspot hypothesis in connection to the New England Seamount Chain, the use of radiometric ages of K-Ar and 40Ar-39Ar were examined (Duncan, 1984). From southeast to northwest there is an increase in seamount construction leading to the northwestward motion of the North American plate over a New England hotspot between 103 Ma and 83 Ma (Duncan, 1984). Fitting the seamount distribution with a volcano migration rate of 4.7cm/year, the ages align with a larger age progression from the Corner Seamounts, on the eastern end (70 to 75 Ma) to the younger White Mountain Igneous Province (100 to 124 Ma) (Duncan, 1984). The age-space relation used does not account for the Older Igneous Province, leaving a gap in the hotspot model (Duncan, 1984).
Through the dating of 26 igneous complexes via K-Ar analysis, it was thereby ruled out that the single hotspot hypothesis can account for the full formation of the WMMS as it does not account for the spread of ages, a non-consistent time transgression from 98 to 238 Ma, nor does it account for the dates appearing to show more episodic activity than continuous (Foland and Faul 1977). The WMMS complexes were hypothesized to have originated along the extension of a transform fault during sea-floor spreading (Foland and Faul 1977).
Arguably, the younger White Mountain Igneous Province and older White Mountain Igneous Province could be initiated and positioned along weak zones of deep-seated fractures, explaining their overlap (McHone, 1981). The overlap seen in mapping of the WMMS can be stress related to the opening of the both the central Atlantic and northern Atlantic and the gradual strain along the zones caused magmatism to decrease (McHone, 1981). The regional uplift as a result of the hotspot movement (Crough, 1981b), can be accounted for by the transfer of heat into the lithosphere by intrusions (McHone, 1981). In an argument against the hypothesis of weakened zones, it is stated there is no global relation between volcanic lineaments and surficial features, the majority of the dated volcanic lineaments show an age progression, midplate volcanism is not known to occur across the same lineaments at separate times, and lastly three major lithospheric faults four separate periods of activation would be needed to account for all features and data (Crough, 1981a).
Elaborating upon the proposed hypothesis of weakened zones due to rifting (McHone, 1981), once the Atlantic had opened, a significant quantity of granitic magma and undersaturated gabbro-diorite-syenite were formed and hypothesized to be a result of melting in the thick crust caused by volatile upwelling or increased heat flow, thus creating the WMMS (McHone and Butler, 1984). The extended nature of the WMMS is proposed to be a result of mantle upwelling along and extensional fracture zone in which the WMMS is a reflection of the orientation and positioning of a deep basement structure parallel to the Connecticut River Valley and Lake Champlain Valley (McHone and Butler, 1984). At the thickest parts of this lower crust, partial melting occurred, crustal thinning and erosion were accelerated by uplift, and the WMMS was emplaced as the deep basement structures were technically active under the influence of mantle convection during rifting (McHone and Butler, 1984).
From geochronological data, a thermal anomaly existed for an extended period of time under the WMMS (Eby et al., 1992). The mantle source, through isotopic dating, matches characteristics similar to that of oceanic island basalt source but determining whether that source a hotspot or from rifting is not known (Eby et al., 1992). In either case, it is proposed that the mantle-derived melts were emplaced into the crust at the base and by fractional crystallization evolved and this stage was interrupted and the magmas were moved to a higher crustal level where later evolution took place (Eby et al., 1992).
The Central Atlantic Magmatic Province (CAMP)which extends to the north and south on either side of the Atlantic Ocean where magmatism occurred at about 200 Ma and in Maritime and New England province (CNE)this magmatism occurred between 225 and 230 Ma (Eby, 2013). This magmatism is immediately followed by the older White Mountain Igneous Province (OWM) as it a distinctly different emplacement of igneous rocks, from about 200 to 160 Ma and then in roughly 122 Ma the Monteregian Hills White Mountain Igneous Province (MHWM),introduced displaying a greater range of diverse rocks (Eby, 2013). The rarity of mafic rocks in the OWN negates any direct comparison with CAMP magmas; although, OWM samples have elemental and isotopic characteristics similar to CNE and MHWM which are drastically different from that of CAMP magmas (Eby, 2013). As mafic rocks are abundant in the MHWM and these magmas have been hypothesized to be derived from a depleted mantle source and are related by degrees of melting and crustal contamination, the same models can be applied to the OWM and CNE (Eby, 2013). It can thus be concluded that the CNE, OWN, and MHWM were all derived from a similar matching magma source and are representative of varying magma compositions related to variations in degrees of partial melting and crustal contamination (Eby, 2013). It is pointed out however, this does not link CAMP magmas to these three as it must come from a separate source magma and has a different history (Eby, 2013).
Using the connection made between OWN, MHWM, and CNE, a step in determining the origin of the WMMS is to determine the origin of the CNE. The CNE magmas may the start of a plume origin for the CAMP magmas; however, because of the lack of relationship between the CAMP and CNE magmas this hypothesis is yet to be resolved (Dorais, 2005). In assuming that the CNE magmas were the initial magmatism in a plume event, then a composition of oceanic island basalts would not be expected; however that is what CNE I has as a composition (Dorais, 2005). Oceanic basalts have been hypothesized to represent the end of plume magma events and thus CNE magmas would be assumed to have to have erupted after that of the plume, not prior (Dorais, 2005). It has been concluded however, that the CNE rocks may represent pre-shield type magmatism prior to CAMP as it matches elemental characteristics of Loihi magmas which were precursors to the shield magmatism in Hawaii (Dorais, 2005). With these conclusions and the connections between OWM, MHWM, and CNE it is possible that the hotspot/mantle plume hypothesis has further support.
It is clear that there is no exact answer to how the White Mountain Magma Series was formed and how it was emplaced into its current positioning. I believe it is fair to say that the hypothesis of fracture zones under the province (Chapman, 1968) has little evidence to be considered a reasonable explanation. As to the debate over whether the WMMS is a result of a hot spot track or rifting due to the opening of the Atlantic, I do not believe there is a concise answer. Both hypotheses have what seems to be logical evidence for support while they also both have flaws and unaccounted for aspects. To determine one origin hypothesis, I believe it is relevant to continue work in looking at the larger picture of the WMMS and how it is similar and different to the series of the CAMP and CNE magmas. If additional connections can be made in terms of composition and dating models then additional progress in terms of origin of both the WMMS and the CNE magmas.
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