Emplacement of the Maidstone and the Black Hill plutons: composite intrusions on the of the Northeast Kingdom Batholith, Vermont

Clark Filip, Johnathon Miller and David Westerman
Department of Geology and Environmental Science
Norwich University

ABSTRACT
The Maidstone and Black Hills plutons are located respectively on the eastern and western margins of the Northeast Kingdom batholith, a Devonian-aged complex in northeastern Vermont that intruded the regionally metamorphosed and deformed rocks of the Connecticut Valley – Gaspé Trough. Recent geologic, petrologic, and geochemical analyses of these two plutons show that they have overall similarities with respect to their geochemical variability and emplacement histories.

Diorite, tonalitic granodiorite, monzogranite, and leucocratic low-Ti monzogranite of occur as discrete zones within the Maidstone pluton; similarly, discrete zones of quartz diorite, low-Ti quartz monzodiorite, and quartz monzogranite to granodiorite make up the Black Hills pluton. In each case, the pluton is composite, with contrasting mineralogy and geochemistry between individual intrusions that were produced by discrete magma pulses. Additionally, the region surrounding the Black Hills body includes discrete intrusions of the same lithologies, along with granodioritic dikes, that make up the main pluton. The composite character of each pluton is particularly well supported by overlapping ranges of silica content accompanied by distinctive abundances of other elements, producing sharp graphical separation on discrimination diagrams. These relationships suggest development of composite plutons in the Northeast Kingdom batholith by sequential emplacement of evolving magma sources, and raise the question as to whether or not this process may be the rule rather than the exception.

INTRODUCTION
The emplacement of granitic magma within the continental crust represents the final stage in the granite-forming process, but is a fundamental process in crustal recycling and growth. Over the last decade, studies of surface exposures of granitoid plutons in a variety of tectonic settings have improved our understanding of magma transfer and accumulation processes (Paterson and Vernon, 1995; McCaffrey and Petford, 1997; Wiebe and Collins, 1998; Miller and Paterson, 1999; McNulty et al., 2000; Molyneux and Hutton, 2000; Miller and Miller, 2002; Mahan et al., 2003; Glazner et al., 2004; Walker et al., 2007). A main achievement in this field has been the progressive replacement of the customary view of granitic magma slowly rising as diapirs by the concept of felsic magma fastly ascending via self-propagating dikes (Petford, 2000).

The feeding of a pluton by dikes can result, over time, in the injection of a number of discrete pulses of magma. In this framework, reconstructions based on several techniques show that some major plutonic systems accumulated in small batches as subhorizontal or subvertical sheets (McNulty et al., 1996; Wiebe and Collins, 1998; Mahan et al., 2003; Coleman et al., 2004). Such a multi-pulse magma input is a common feature shared by plutonic and volcanic systems, and deciphering the intrusive record of magma systems is thus essential also for the understanding the relationship between large felsic eruptions and the long-term storage of magma in subterranean reservoirs (Bachmann et al., 2007; de Silva and Gosnold, 2007; Miller et al., 2007). Therefore, attempts to identify magma batches within a pluton are crucial to constrain the possible episodic nature of pluton growth, the rates of magma input over time, and the workings of volcanic plumbing systems.granites build crust – magma generation, separation, transport and emplacement – Northern Vermont has widespread occurrences of granite and related rocks that represent enormous volumes of molten rock trapped in the Earth’s crust about 380 million years ago as Africa snuggled tightly against North America. This study addresses the origin of the variations that occur within what have been mapped as individual plutons as well as regions characterized by sets of similar plutons.

GEOLOGIC SETTING

Figure 1. Schematic geologic map (from Coish, pers. comm, 2007), showing the location of the two studied plutonic complexes within the Northeast Kingdom batholith.(Click images to enlarge.)

Vermont exposes basement, rift, platform, and arc sequences on the western side of the Appalachian orogen. In eastern New York and western Vermont, Precambrian basement is exposed as the Adirondack massif and as inliers – the Green Mountain and Lincoln massifs. Cover units (west to east) include early Paleozoic carbonate platform, rift facies represented by the Green Mountain tectonic slices, oceanic accretionary prism rocks in the Rowe slices, fore-arc and arc deposits in the Moretown & Hawley slices, Siluro-Devonian Waits River and Gile Mountain Formations in eastern Vermont, and finally portions of the Bronson Hill arc terranes (Doll et al., 1961; Stanley and Ratcliffe, 1985). Granitic plutons of purported Devonian age intruded the Waits River and Gile Mountain Formations.
Western and central Vermont record the evolution of the Laurentian continental margin from a late Precambrian rift valley to early Paleozoic continental shelf sequence. The continental shelf bordered the western margin of the Iapetus Ocean. Eastward subduction in that ocean led to construction of either one long-lived volcanic arc (Stanley and Ratcliffe, 1985) or a short-lived arc followed by subduction flip and a second arc (Karabinos et al., 1998; Moench and Aleinikoff, 2002). In both cases the arc collided with the Laurentian continent, and formations in western and central Vermont were assembled into westward-directed thrust slices (Green Mountain, Rowe, and Moretown & Hawley slices) on the continental edge (Stanley and Ratcliffe, 1985). In northeastern Vermont, the Connecticut Valley Trough is occupied by mostly low-grade metamorphosed pelitic and quartzitic units of the Gile Mountain Formation that overlie calcareous quartzitic and pelitic units of the Waits River Formation. Siluro-Devonian ages are assigned based on Early Devonian plant fossils in the Gile Mountain Formation (Hueber et al., 1990) and a U-Pb zircon age of 423 Ma from a dike cutting the Standing Pond Volcanic member of the Waits River Formation (Aleinikoff and Karabinos, 1990). Ages may range regionally from 438 to 394 Ma (Tremblay and Pinet, 2005). All of eastern (and part of western) Vermont is thought to have been affected by the Acadian Orogeny around 380 – 390 million years ago, when amalgamated Laurentia collided with peri-Gondwanan/Ganderian terranes.

Granitoid plutons of this study are part of the Northeast Kingdom batholith (Ayuso and Arth, 1992; Arth and Ayuso, 1997) which intruded both the deformed Waits River and Gile Mountain Formations near or following the end of the Acadian Orogeny. At that time, they superimposed distinct contact aureoles on the regional deformational fabric, which may have been formed at around 390 Ma (Bradley et al., 2000). Many of these plutons have no tectonic fabric but some reveal foliations along faults may be Acadian in age (Hannula et al., 1996). In general terms then, most of the granites are younger than 390 Ma, but there is a dearth of published, high-precision U-Pb ages on granitoids in northern Vermont.

BLACK HILLS COMPLEX

Petrography

Figure 2. Geologic map of Black Hill complex, see table for correlation of sample locations with rock unit.

Petrographic and physical appearance of samples, supported by geochemical analysis, reveals five different units of granitic rock in the area of study.
1. Black Hills monzogranite is a medium-grained, gray rock (7-10% biotite) that makes up more than half of the Black Hills pluton, having produced distinctive contact metamorphic effects where it baked the surrounding country rock to form dense graphitic schist, with or without the minerals garnet and staurolite.
2. Black Hills quartz monzodiorite is distinctive, but has limited distribution. It is found in one known location along the northeast margin of the main pluton, and is distinguished by its low quartz content and high color index (melanocratic).
3. Caspian Lake granodiorite, exposed at Caspian Lake but making up the northern two-fifths of the Black Hills pluton, is medium to coarse grained with a low color index (leucocratic).
4. Granodioritic dikes are distinguished by their fine-grained, porphyritic textures.
5. Craftsbury monzogranite contains obvious orbicules very rich in concentrically distributed biotite, its most distinguishing feature. The matrix of this rock is, in hand sample, nearly identical to the
overall texture of the Black Hills monzogranite.

Figure 3. Units of the Black Hills complex and associated intrusions.

Geochemistry

The table below presents results of whole rock analyses, along with calculated normative mineral abundances. The Black Hills monzogranite, the Caspian Lake granodiorite, and the Black Hills dikes are three distinct systems in the Black Hills complex as recognized by the fact that they plot separately when the main indicator of fractionation (SiO2) is plotted against TiO2, Fe2O3, MgO, V, and Cr on binary diagrams.
1. Black Hills monzogranite is intermediate in all ranges of chemical composition and correlates most strongly with the granodioritic dikes disseminated through the study area.
2. The Caspian Lake granodiorite is the most evolved unit, displaying overall higher weight percents of Na2O and lower concentrations of Fe2O3 and MgO, with consistent chemical signatures for both exposures separated localities.
3. The Craftsbury monzogranite, characterized by a unique orbicular structure, is relatively poor in alkalis and enriched in iron and magnesium. Its geochemistry contrasts strongly with that of the Black Hills monzodiorite unit which represents the least evolved magma in the complex, with low SiO2 plus total alkalis and high ferromagnesium content.

Table 1. Whole rock analyses of the Black Hills complex and associated intrusions.

Figure 4. Geochemical diagrams illustrating the discrete character of intrusive units, as well as their commonalities.

Figure 5. AFM diagram of Black Hills and associated units; legend as in Figure 4.

MAIDSTONE PLUTON

Petrography

Figure 6.

Field studies initially indicated that the Maidstone pluton consisted of two separate units:
1.     felsic feldspar-rich granite, and
2.     diorite/quartz diorite rich in  hornblende and biotite.

Staining and petrographic analysis expanded the number of identifiable units to at least  three. The granite was subdivided into:
1.     two-feldspar granite rich in quartz and low in biotite,
2.     tonalite that is finer-grained, contains only plagioclase feldspar, and is relatively rich in quartz with an intermediate biotite content, and
3.     mafic biotite- and hornblende-rich diorite/quartz diorite, also devoid of significant alkali feldspar.

Further subdivision of the granite was not possible on the basis of petrographic criteria alone.

Figure 7. Lithic units of the Maidstone pluton and accompanying AFM diagram.

Geochemistry

Geochemical analyses for major and trace elements of the Maidstone units are presented below, along with calculated norms. Plots on discrimination diagrams confirm the recognition of the units identified by field and petrographic analyses. However, the most voluminous unit is revealed by the geochemistry to be two separate intrusions.

Whole rock analyses of Maidstone lithologie

The mafic to intermediate rocks of the diorite/quartz diorite unit have SiO2 values in the range of 50 – 60 wt%, with high MgO, Fe2O3 and TiO2 contents and relatively low alkali contents.

Tonalitic rocks consistently are discriminated by their intermediate compositions. Most significant is the non-linear trend in AFM plot indicating that the tonolitic magma (blue) was not evolving directly to the granitic units (red and purple).

Finally, the main mass of the Maidstone pluton is recognized as two distinct intrusions. The northeastern mass (red) is slightly less evolved with lower combined SiO2 and alkalis, and high MgO, Fe2O3 and TiO2 contents.

Variation diagrams illistrating geochemical characteristics of Maidstone units.

BLACK HILLS DISCUSSION
The intrusive rocks of the Black Hills complex have distinctive geochemical signatures that suggest that each unit evolved from its own source area of magma. The Black Hills monzogranite, the Caspian Lake granodiorite, and the granodiorite dikes show fractionation trends over the same range of silica content. Discrete emplacement of each dike indicates their source region was evolving with loss of iron and magnesium and enrichment in alkalis. The larger masses, however, may have developed their internal variations by crystallization in situ as a result of fractional crystallization and crystal settling. This study highlights the value of ongoing geologic mapping.

MAIDSTONE DISCUSSION
Analyses of rocks from the Maidstone pluton suggest that it is composed of four separate units, and that these units did not evolve following after emplacement of a single magma. Progressive evolution of a rock series generally occurs with silica enrichment, but the tonalite, granite and leucogranite units overlap considerably in their silica content. In the triangular AFM diagram , there is a sharp offset in the trend between the tonalite (blue) and the two granites (red and purple). Taken together, these data indicate that the evolution of the magma occurred at depth, and that emplacement of pulses at a single magmatic center were separated in time.

Distribution of the rock types as seen in the map above supports this idea, since each unit is confined to it’s own geographic region. We visualize a single magma source below that evolved throughout the time period of emplacement by loss of mafic minerals and calcic plagioclase, as illustrated in the model below.

CONCLUSIONS
Study of two small plutonic complexes in the Northeast Kingdom batholith of Vermont show that previously undifferentiated bodies are in each case composite in character, reflecting emplacement of multiple pulses of magma. In the case of the Black Hills along the western margin of the Connecticut Valley trough, the largest intrusion, the Black Hills pluton proper, is composite. It consists dominantly of magma derived a source similar to that which formed the granodiorite dikes of the system, but the same pathway transported Caspian Lake magma. Internal variations within these masses are minimal and may represent local differentiation processes that occurred in situ, but for the most part, magmatic variations reflect deep-seated causes.

The Maidstone pluton on the eastern margin of the Connecticut Valley trough is composite having formed from four pulses of increasingly evolved magma, with each batch representing processes controlled well below the level of emplacement. Each successive intrusion was larger than the previous one, producing a composite system by repeated underplating of magma derived from an evolving source area. This study suggests that detailed mapping in combination with petrographic and geochemical analysis can place constraints on further work directed at the full spectrum of magma generation, extraction, transport and emplacement, and supports the notion that plutons are commonly built of successive accumulation of discrete magma batches.

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