Master’s Research Paper, Part 1

This is a reproduction of the first part of the paper I wrote for my Master of Arts degree in geology.

Edit 03/31/2021 – To be honest, I didn’t put the second half into this post because I couldn’t figure out how best to display the data.  So, here’s a link to the  Full Master’s Thesis!

On the Magmatic Plumbing and Differentiation of a Shallow Mafic Intrusive System: Morgantown Pluton, its Birdsboro Dike, and the Nearby Jacksonwald Syncline, Newark Basin, Pennsylvania, U.S.A.

Peter Martinson
Candidate for Master of Arts
West Chester University of Pennsylvania
1 December 2014

ABSTRACT

Geochemical, petrological, and structural clues are used to determine how magma filled and differentiated within the main sill of Morgantown Pluton, its Birdsboro Dike, and the nearby Jacksonwald Syncline. Whole rock geochemistry and ESEM-EDS analysis of zoning patterns shows that 1) basalt flows at the top of the Jacksonwald Syncline experienced vertical differentiation, 2) Birdsboro Dike experienced horizontal differentiation that cannot be explained through simple fractional crystallization of the original liquid therein, and 3) the southern sill of Morgantown Pluton was a site of accumulation of orthopyroxene phenocrysts as well as filter pressing-driven expulsion of evolved liquid. Measurement of plagioclase-plagioclase-pyroxene dihedral angles within samples from the southern sill (~97° at the bottom to ~92° at the top) show that the bottom cooled slower than the top of the sill, suggesting magma recharges from the bottom. Preliminary measurement of plagioclase aspect ratios in one sample from the top sill (2.79±0.05) indicates a cooling time of about 30 years. Models of crystallization pathways of parental liquid compositions using alphaMELTS suggest the magma that intruded the system was not a pure liquid, but a slurry that contained already crystallized orthopyroxene phenocrysts which had formed in the magma prior to intrusion. A specific hypothesis evaluated is as follows: Magma intruded the southernmost portion of Morgantown Pluton first, but the magma did not sit long in this lowest sheet before encountering a Paleozoic tear fault discontinuity, ascending there into the Birdsboro Dike, and finally erupting onto the Earth’s surface as the Jacksonwald Lava. After the initial intrusion was complete, magma differentiated mainly as a result of gravitational settling of the orthopyroxene phenocrysts down the dike back into the sill, collapse of roof sections from the sill, and filter pressing of evolved liquid from the sill up into the dike.

I. INTRODUCTION

Morgantown Pluton is a layered mafic intrusion which lies at the southern corner of Berks County, PA. It is one part of a much larger complex of 200±4 Ma mafic intrusions (Marzoli et al. 1999; Blackburn et al. 2013) that spreads across the Atlantic coasts of North and South America, North Africa, and Europe, collectively called the Central Atlantic Magmatic Province (CAMP). In Eastern North America, the CAMP is represented as a network of igneous sheets and dikes that crop out in basins of Triassic-aged sedimentary rock and conglomerate. These Mesozoic basins are roughly parallel to the Atlantic coast, and were formed during the initial rifting of Pangaea and the opening of the Atlantic Ocean. The intrusion of enormous masses of diabase into the basins coincides precisely with the end-Triassic mass turnover of marine fauna identified by Raup & Sepkoski (1982), one of the big five Phanerozoic mass extinctions (Blackburn et al. 2013).

The intrusions within the Eastern North America (ENA) portion of the CAMP have the surface expression of diabase rings. Drilling within the rings has established that the intrusions are actually continuous sheets, perhaps comparable to the saucer-shaped sills of Karoo Basin, South Africa (Hotz 1952; Polteau et al. 2008). The upturned edges of saucer-shaped sills have been shown to be capable of delivering flood basalt to planetary surfaces, but many dikes are present as well in the ENA basins. The strike of these dikes is not random, but rather has been found to vary systematically along the basins – northeast-southwest in the north, to northwest-southeast in the south (King 1961, 1971).

Geochemical analysis of dikes, chill margins, and basalt flows was used to demonstrate that the rocks share a common tholeiite ancestry (Weigand & Ragland 1970; Smith et al. 1975). They can be broken into three subfamilies: 1) an olivine-normative tholeiite, 2) a high-Ti quartz-normative tholeiite (1.0-1.2 wt% TiO2), and 3) a low-Ti quartz-normative tholeiite (0.65-0.85 wt% TiO2). Local variations within any diabase body can deviate strongly from the basic types, though the variations came about through differentiation of a magma matching one of the types. The high-Ti quartz normative (HTQ) tend to be the most compositionally diverse, containing orthopyroxene cumulates, iron rich ferrogabbros, and granophyres, in addition to typical diabase.

Perhaps the most famous of the HTQ mafic sheets is the Palisades Sill, a beautifully exposed, 300 m thick, 70 km long tablet of black rock rising, cliff like, along the west bank of the Hudson River. Based on an analysis of layering within the sill, Shirley (1987) proposed that differentiation within the Palisades magma was driven by plumes of crystallizing mush that fell from the roof to the floor. The resulting cumulate pile compacted and drove residual liquid out and upwards. Further evidence for compaction and recrystallization of roof-derived plumes that resulted in modal layering was attained by Dickson & Philpotts (2001) and Dickson (2006). Besides vertical differentiation, Gottfried & Froelich (1985) has documented that lateral differentiation within the sheets is more common, and more extreme. Corroborating evidence was found for the York Haven sheet, just west of Morgantown Pluton, by Mangan et al. (1975). Several roof-to-floor sections obtained throughout the York Haven sheet revealed three major regions – an orthopyroxene-cumulate dominated region to the southeast, an iron and granophyre rich region towards the northwest, and more typical diabase in between. This lateral differentiation has been interpreted as due to lateral flow segregation of orthopyroxene and other early cumulate phases during emplacement, followed by gravitational settling, and ending with late-term hydrothermal alteration. One implication of this theory is that orthopyroxene cumulate zones are located near the original conduits from which deep magma intruded the sedimentary basins.

Morgantown Pluton, though no less interesting than its two siblings, lies mostly hidden from view, both physically and in the literature. Pennsylvania geologic maps made before 1980 do not even present the pluton as one connected body (Smith 1893). Outcrops of the Morgantown Pluton are scattered among sometimes precarious roadcuts and a few diabase quarries. Otherwise, evidence of its presence is betrayed by weathered boulders dug up by landowners and construction crews. There are no known outcrops that run from roof to floor. However, there are exposures where diabase meets country rock at a quarry in the northern part of the pluton, but this portion of the pluton is a nearly vertical chamber called the Birdsboro dike. At most other locations, elevation of outcrop above or below the contacts must be estimated.

The pluton lies quite close to a small basalt flow, though their relationship is somewhat cryptic. The Palisades Sill outcrop is also located to the south and east of a series of extensive basalt flows, collectively called the Watchung Basalt. Based on geochemical affinities and an observed point of connection, some believe magma that fed the basalt flows first travelled through the sill (Puffer et al. 2012). The basalt flow near the Morgantown Pluton has no point of physical contact with other subsurface diabase, and lies atop a stack of related intrusions within the plunging Jacksonwald Syncline (Schlische & Olsen 1988).

Several aspects of Morgantown Pluton and the nearby Jacksonwald Syncline render them unique among their diabase siblings. The fold axis of the syncline is almost parallel to the Birdsboro Dike. The dike itself is peculiar in that it runs perpendicular to the average NE-SW dike trends in the Newark Basin (King 1961), and is over 200 m thick along its entirety. The dike marks the southwestern boundary of the Newark Basin, and Morgantown Pluton lies in a transition zone between that basin and the so-called Narrow Neck region. These facts present themselves as indications that an understanding of how Morgantown Pluton formed and how it is related to the syncline is necessary for understanding the formation of the Newark Basin as a whole, and could provide insights into the development of tectonic basins in general.

For the present report, petrological, geochemical, and structural evidence will be used to evaluate a hypothesis about the formation and relationship between the southern portion of Morgantown pluton, Birdsboro Dike, and the Jacksonwald basalt. That hypothesis is as follows: Before the magma which intruded the southern sill of Morgantown Pluton had time to begin crystallizing, it spread up Birdsboro Dike, an ancient tear fault, and spilled onto the surface of the Jacksonwald flood basalt. Subsequent inputs of magma were confined to the sill, and most chemical differentiation was due to the combination of crystal settling and consequent liquid displacement. The presentation of data which will help evaluate this hypothesis will be preceded by a more extensive description of the geography and geological setting of Morgantown Pluton, the Jacksonwald Syncline, and the enclosing Newark Basin.

II. GEOLOGIC SETTING

Figure 1: Eastern North American Mesozoic Basin map

Morgantown Pluton and the Jacksonwald Syncline inhabit the southern end of the Newark Basin (Figure 1). More specifically, the northeastern border of the pluton is the ~20 km long, 200-300 m thick Birdsboro Dike, which itself forms the southwestern boundary of the basin (Figure 2). Thus, the Jacksonwald Syncline lies inside the Newark Basin, while the rest of the Morgantown Pluton lies in a transition region between the Newark Basin and its neighbor, the Narrow Neck. Let us first examine the geography of the pluton and the syncline, and then look at the structure of the basin surrounding the two.

Figure 2: Map of Morgantown Pluton and the Jacksonwald Syncline

Birdsboro Dike strikes an average of N50W and dips about 80° SW. The dike runs from just south of Reading, PA to a little east of St. Peters about 20 km to the southeast. Here, the pluton takes a sharp turn almost due west into a broad, 200-300 m thick sill that dips about 24° towards the north (Wood, 1980). This sill runs continuously for about 15 km west-southwest, until breaking into a somewhat chaotic region which steps alternating west and north for another 15 km to just south of Knauers, PA, where the pluton widens into what may be an offshoot subsill. It may be that this chaotic region represents a series of short sills interconnected by short dikes which allowed the magma to jog upsection to this offshoot subsill. The pluton continues north from the subsill to just east of Fritztown, PA, where it again takes a sharp turn towards the east. This portion of the pluton has been described as either a steeply south dipping sheet or a shallow dike, based both on its uniformity as well as on its aeromagnetic signature (MacLaghlan et al. 1972). This portion ends when it reaches the Schuylkill River, which is also where Birdsboro Dike begins.

The axis of the Jacksonwald Syncline and its associated igneous bodies lies about 5 km northeast of Birdsboro Dike. The syncline, which is parallel to the dike, and dips about 17° NW, contains a lava flow and three diabase intrusions. The first diabase intrusion lies concentric to the lava flow with about 1 km larger radius. The second diabase intrusion, Monacacy Hill, lies about 6 km towards the east-southeast of the first. The Monacacy Hill pluton appears more tabular and less folded than the prior two igneous bodies. The third and last diabase intrusion, Rattlesnake Pluton, lies about 4 km east of Monacacy Hill. This intrusion is flat along its upper margin, and curved along its lower, which suggests it was intruded into accommodation space already created by the folding of the syncline (Schlische & Olsen 1988).

All known exposures of Morgantown Pluton exhibit some form of layering, about which more will be described below. However, one important fact derived from the layering should be stated here. Srogi et al. (2010) has shown that layering found on the walls of the Pennsylvania Granite Quarry, in the middle of the southern sill, dip conspicuously towards the north about 20°. Similar subhorizontal layering found throughout the pluton also exhibit a range of 15-20° dip either NNW or NNE. This uniformity of orientation suggests that the layering formed normal to the Earth’s gravitational field, and that the pluton was tipped towards the north after intrusion, and after solidification. The utility of this knowledge is that the northernmost portions of the pluton formed at a paleodepth about 5-6 km above the southernmost portions. Similarly, the plunge of the Jacksonwald Syncline places the base of the Rattlesnake Pluton about 4 km below the base of the basalt flow, and thus below the paleosurface.

This knowledge allows us to give a back-of-the-envelope estimate of the total volume of diabase within Morgantown Pluton, given a few reasonable assumptions. Assume that the southern sill is a disk exposed along its diameter. With a diameter of 15 km, thickness of 300 m, its volume will be 53 km3. Assume Birdsboro Dike is a tablet exposed along its long diagonal. If the diagonal is 20 km, thickness is 300 m and depth is 5 km, the volume will be 29 km3. The chaotic portion along the southwestern margin can be broken into several small sills and short dikes, which ends up having about the same volume as Birdsboro Dike, 29 km3. The northern portion is assumed to be a dike about 5 km deep, 300 m wide, and 12 km long, giving it a volume of 18 km3. Altogether, this gives Morgantown Pluton a rough volume of about 130 km3.

STRUCTURAL CONSIDERATIONS

Birdsboro Dike is peculiar because it is a dike as thick as a typical Newark Basin sheet (200-300 m), but what renders it so conspicuous is its orientation. King (1961) demonstrated that the dikes in and around the Newark Basin strike northeast, which reflects the regional stresses during the period of rifting. Birdsboro Dike lies almost orthogonal to this stress field. A cursory examination of the geologic map (Figure 1) reveals the striking alignment of the dike as a continuation of the south-east margin of the Newark Basin. Comparatively striking is the trend of the Jacksonwald Syncline, which runs exactly parallel to the Birdsboro Dike, again suggesting a structural kinship. The orientation of these two features suggests that the magma was responding to some factors other than the typical stresses associated with rifting. It may be instructive in our investigation of the Morgantown system’s origin and plumbing to summarize the current structural thinking about this region.

Figure 3: Model of Newark Basin’s evolution, from Schlische (1992)

The most complete working model of basin evolution comes from Schlische (1992). In his view, the ~200 km long Newark Basin is a half-graben bounded on its north-west edge by a system of normal faults that dip towards the Atlantic coast. The basin began its life as a system of smaller synclines that plunged northwest and terminated against normal faults. These synclines grew and eventually merged into the larger half-graben (Figure 3). It may be that the local synclines observed in the Newark Basin today, like the Jacksonwald Syncline, were the original nuclei of the greater basin. As the basin floor tilted down against the border fault system, and the fault footwalls isostatically rose, sediment filled in from both sides. Repeated debris flows dumped mainly limestone fanglomerate at the base of the border fault system. Along the south-east margin lay the ridge of mountains formed during the Appalachian orogenesis, which delivered seasonal fluvial sediments into the basin. As the basin grew, fluvial sediment gave way to lacustrine deposition. Schlische suggests that local synclines and anticlines within the main basin (Figure 4) may have formed as short-wavelength corrugations due to both compression along the surface parallel to the basin margin, as well as to differential slip along the border fault system. After several kilometers of sediment accumulated, early Jurassic intrusive diabase dissected the sediment above the thinned basin crust. The entrainment of both diabase and lavas within the synclines suggests that the igneous event occurred long before the conclusion of folding. The extreme ends of the major basins served as axes of rotation that allowed the intrusion of magmatic dikes as accommodation features.

Figure 4: Synclines and anticlines at the southwestern end of Newark Basin. Note the Jacksonwald Syncline and Birdsboro Dike on the left. From Schlische (1992).

This construction offers at least two controls on the diabase that interest us in the present study. First, the model suggests that several of the sills and lava flows were folded along with the synclines, and thus were originally emplaced flat and only developed their currently observed curvature later. Schlische (1992; also Schlische and Olsen 1988) specifically treats the four igneous portions of the Jacksonwald syncline as follows. The lava flow and the intrusive diabase immediately below exhibit no thickening, or “ponding,” near the synclinal axis, which suggests that they were emplaced as relatively flat entities and folded afterwards. The two deeper intrusions, however, exhibit clear ponding, which suggests that they were emplaced as phacoliths into weakened zones along an already formed synclinal hinge. Second, the model suggests that Newark Basin as a whole functions as a long-wavelength plunging syncline, and that either end of this syncline could be host to steeply dipping igneous dikes intruded as accommodation dikes. While the northern end of the Newark Basin may or may not contain a steeply dipping dike (Puffer et al. 2009, Schlische 1992), the southern end certainly does – the Birdsboro Dike.

Figure 5: The Schuylkill Tectonic Zone is the lightly shaded region between the red arrows. It runs between the Grenville basement rocks. From Wise (2014).

In broader context, tectonic processes pertinent to the formation of the Morgantown Pluton and the Jacksonwald syncline may predate the formation of the Newark Basin, and possibly control the basin’s extent. Wise & Faill (2002) define the Schuylkill Tectonic Zone (STZ) as an 8-10 km wide swath around the Schuylkill River roughly parallel to the Birdsboro dike and Jacksonwald Syncline, running from the northern edge of the Great Valley to the southeast edge of the Honeybrook formation (Figure 5). The abrupt offset of Silurian rocks at the north (the “plunge” north of Morgantown Pluton) as well as the isolation of the Little South Mountain Precambrian klippe suggest that this region is a broad right lateral transform zone. Wise (2013, 2014, personal communication) suggests that the STZ represents a reactivated tear fault inherited from the Paleozoic Alleghanian orogenesis, and that Birdsboro Dike itself may be a trace of this ancient fault. Wise & Faill (2002) demonstrate that a related pattern of seismicity called the Lancaster Seismic Zone (Armbruster and Seeber 1987; Figure 6), the most active seismic zone east of the New Madrid Seismic Zone, betrays the existence of the southern edge of a northern thrust block of Alleghanian origin which lies atop a deeper Alleghanian thrust block. This deeper block’s basement becomes exposed west of the Gettysburg Basin as the South Mountain formation, while the shallower basement rock is exposed as Little South Mountain, the Reading Prong, and the Honeybrook Upland. Wise (2014) suggests that basin dip and width may be controlled by the presence of this basement rock, since the steepest dipping and most narrow basin, the Narrow Neck, lies within the transition between upper block and lower block.

Figure 6: Earthquakes of the Lancaster Seismic Zone. From Wise and Faill (2002).

Figure 7: Hammer Creek formation is a conglomeratic sedimentary rock located within and just southeast of Morgantown Pluton, represented in light green in this figure.

Figure 8: Representation of Birdsboro Dike as a North-dipping normal fault. From Schlische (1992).

The existence of a hitherto unrecognized tear fault beneath the Birdsboro Dike may offer a control on length of the Newark Basin, as it could be the hinge against which the basin was allowed to slip. Such is suggested by sedimentary relations across Birdsboro Dike. South of the dike (inside Morgantown Pluton), the Hammer Creek sedimentary formation is present almost everywhere, while north of the dike the Hammer Creek crops out only at the southeastern end, underneath the younger Passaic Formation (Figure 7). This juxtaposition could occur if the sediments north of the dike dip northwestward more steeply than the southern sediments. This geometry would suggest that the region north of the dike is dropped down relative to the region south. Schlische apparently saw as much – a cross section in Schlische (1992) represents Birdsboro Dike as a steeply dipping normal fault with the down-dropped block on the north side, though nothing this clear is stated in the text (Figure 8).
Now, let us turn to a description of the petrology and rock types found within these igneous bodies.

ROCK DESCRIPTIONS

The diabase and basalt in Morgantown Pluton and the Jacksonwald Syncline generally correspond to the HTQ, York Haven type of tholeiite (Smith et al. 1975, Gottfried et al. 1991d). Chemical compositions of specific samples used in the present study will be detailed below, but in general, the chill margins have a TiO2 wt% of 1.15-1.23, and the lava has TiO2 wt% of 1.07-1.17. Within the pluton interiors, TiO2 can go down to 0.66 wt% (eg. southern sill), and up as high as 3.20 wt% (eg. the dike), though these wide variations likely reflect differentiation processes within the chambers. The diabase is dominated by intergranuar to subophitic grains of plagioclase and pyroxene, with late interstitial phases of biotite, quartz, K-feldspar, granophyre, and Fe-Ti oxides. Especially in the northern/upper portions of Morgantown pluton extensive hydrothermal alteration of pyroxene to amphibole and plagioclase to sericite occurs.

Pyroxene occurs throughout the pluton and lava as the clinopyroxenes augite (Ca > ~20 wt%) and pigeonite (5 wt% < Ca < 15 wt%). In the Jacksonwald Lava, augite phenocrysts are far more abundant towards the floor than the roof, though this has not yet been quantified. In Birdsboro Dike, the modal proportion of pyroxene to plagioclase is 22:50 (px:plag), while in the southern sill, the modal proportion is 53:42 (px:plag), where the pyroxene is divided between orthopyroxene and clinopyroxenes in the modal proportion 19:34 (opx:cpx). Euhedral, cumulate orthopyroxene is typically only found in the southern portions of Morgantown Pluton and the lower Jacksonwald plutons, but can reach almost 20 modal percent (26 wt%) in those regions. To date, unambiguous orthopyroxene phenocrysts outside of the southern portions have been found only in the possible subsill south of Knauers, where it only reaches 2% by volume in samples studied. Orthopyroxene found elsewhere takes the form of so-called inverted pigeonite, a product of exsolution from pigeonites, and never as cumulate phenocrysts.

Chemical zoning of phenocrysts reflect the changing composition of a crystallizing magma. Zoning patterns are witnessed in both the pyroxenes and the plagioclase. Augite zoning ranges from about 79% to 55% enstatitic (Mg/(Mg+Fe) %). In the interior of Birdsboro Dike, this can get down to En30. Plagioclase compositions range from about 80% to about 40% anorthitic (Ca/(Ca+Na) %), but can get down to An25 in the dike interior. Based on zoning patterns, two distinct families of plagioclase have been recognized to exist in the dike and the sill of Morgantown Pluton. In the first family, normally zoned plagioclase, the highest anorthite composition is at the core and the lowest is at the rim. In the second family, oscillatory or reverse zoned plagioclase, lower anorthite cores (An65-An75) are surrounded by one or more rings of higher anorthite zones (~An80) which interzone with low anorthite zones. These phenocrysts are ultimately rimmed with anorthite compositions that compare with those of the first family (Srogi et al. 2010).

Figure 9: An oscillatory zoned plagioclase phenocryst from the Southern Sill (PAGQ), imaged with the ESEM-EDS. Brighter colors mean lower Na, darker colors mean higher Na.

Layering within the pluton takes several forms (Srogi et al. 2010). In the southern sill, layering is defined by concentrations of plagioclase that form light-colored layers ranging from a few millimeters to a few centimeters thick that are immediately underlain by orthopyroxene rich, mafic layers. These subhorizontal layers dip north-northeast at ~20° on average, and appear to be curving and to branch or intersect with other plagioclase-rich layers. The cause of this layering is likely to be similar to that proposed for the Palisades Sill, in which rafts of crystallizing mush from the roof region sink to the floor (Dickson & Philpotts 2001). These subhorizontal layers are turned slightly upwards where they are crosscut by subvertical, mafic channels. It is believed that the subhorizontal layering originally exhibited an average dip of 0°, and that the current orientation was formed through post-magmatic tilting of the entire pluton towards the north. In Birdsboro Dike, layering is rhythmic and steeply dipping, parallel to dike margins. The layers are of approximately equal thickness and consistent in orientation and thickness over several meters. The cause of this layering is as-yet unknown, though it may have to do with thermal patterns set up as a byproduct of crystallization. Layering has also been found near one of the few exposed roof contacts in the western subsill portion of the pluton, and takes the form of repeated fine-grained selvages immediately underlain by coarse-grained granophyric lenses that grade back into medium-grained diabase (Martinson & Srogi 2014). These layers also exhibit a northward dip of ~20°, again suggesting post-solidification tilt of the entire pluton. It is quite likely that these layers were formed by a combination of rising evolved magma segregations and sudden quenching by outgassing events through the roof.

To be continued…