Bedrock geology

BEDROCK GEOLOGY OF THE EASTERN WHITE MOUNTAIN BATHOLITH, NORTH CONWAY AREA, NEW HAMPSHIRE

  • by John W. Creasy, Department of Geology, Bates College, Lewiston, ME 04240
  • John P. Fitzgerald, Haley & Aldrich, 58 Charles Street, Cambridge, MA 02141

THE WHITE MOUNTAIN IGNEOUS PROVINCE

The White Mountain igneous province (magma series of Billings, 1934) consists of plutons, ring complexes, and volcanics emplaced along a NNE trend across New England (Figure 1). Four petrographic associations are recognized (Creasy, 1974; Eby, 1987): (1) alkali syenite-quartz syenite-granite; (2) subaluminous biotite granite; (3) gabbro-diorite-monzonite and; (4) syenite-nepheline syenite. The igneous activity is largely confined to two periods, 200-165 Ma and 130-110 Ma (Eby and others, 1992). These t wo major periods of igneous activity are related by McHone and Butler (1984) to the opening of the North Atlantic Ocean. The reader is referred to Eby (1987) for an overview of the White Mountain igneous province.The older White Mountain igneous province is dominated by silica-oversaturated subaluminous to peralkaline rocks of associafion 1, including the White Mountain batholith. Two minor nepheline-bearing intrusions occur at Red Hill, New Hampshire, and Rattle snake Mountain, Maine. These two occurrences together with nepheline-bearing intrusions of the younger White Mountain province define a narrow zone that strikes at high angle to the NNE trend of the overall province (Figure 1; Creasy, 1989). To the nort h of this zone are found the large composite plutons and batholith of the older province; to the south only a few small scattered plutons of this age are present. In contrast, nearly all plutons of the younger White Mountain province are found to the sou th of this zone.

The Monteregian Hills and younger White Mountain igneous provinces represent the last period of igneous acfivity in New England (130-100 Ma). The bulk of the magmatism occurred ca. 125 Ma, but younger ages have been obtained for Little Rattlesnake (114 Ma, Foland and Faul, 1977) and Cuttingsville (100 Ma, Armstrong and Stump, 1971). Plutons emplaced to the west of Logan’s line consist largely of mafic alkaline suites, many of which are nepheline norrnative. To the east of Logan’s line, felsic ro cks are much more important components of the intrusions and silica-undersaturated rocks are not found. Some of these younger plutons show ring-like structures (Ossipee and Pawtuckaway) while others appear to he small plugs (e.g. Little Rattlesnake, Ascu tney, and Tripyramid). In most cases the most evolved rocks are syenites and quartz syenites, but biofite granites are found at Ossipee and Merrymeeting Lake.

This field excursion illustrates the White Mountain batholith [associations 1 and 2 ahove] which comprises about 50% of the total areal extent of the older White Mountain igneous province. The material in this field guide previously appeared in Creasy and Eby (1993).

THE WHITE MOUNTAIN BATHOLITH - INTRODUCTION

The White Mountain batholith (Figure 2; see also Hatch and Moench, 1984) is a composite of several overlapping centers of felsic magmatism. Individual centers are strikingly defined by composite ring dikes of porphyritic quartz syenite. Thick sections of rhyolitic crystal tuffs, breccias, and subvolcanic granite porphyry are partially circumscribed by the ring dikes. A mosaic of subalkaline to perakaline silica-oversaturated plutons intrude these centers and provide areal continuity to the batholith. Dist ribution of porphyrific, miarolitic, and aplitic textures indicate that the roofs of several plutons are partially intact.

The geology of the White Mountain batholith is described by Billings (1928), Billings and Williams (1935), Creasy (1974), Davie (1975), Eby and others (1992), Fitzgerald (1986), Henderson and others (1977), Moke (1946), Osberg and others (1978), Parnell ( 1975), Smith and others (1939), Wilson (1969) and, Wood (1975). Granites, quartz syenites, and syenites account for about 97% of the 1,000 km2 area of the batholith; volcanic rocks of similar composition account for the remainder. Pink, medium-grained s ubalkaline biotite granite (the Conway Granite) and a green, medium-grained subalkaline to peralkaline amphibole granite (the Mount Osceola Granite) comprise 80% of the batholith. Medium-grained sub-alkaline to peralkaline amphibole syenites and quartz s yenites are widely distributed and are similar in occurrence, texture, and mineralogy to the Mt. Osceola. Distinctive porphyritic quartz syenite occurs in ring dikes in the western (the Mount Garfield) and the eastern (the Albany) halves of the batholith . Fine-grained syenite occurs in isolated outcrops spatially associated with the ring dikes.

Figure 1. The White Mountain igneous province (Creasy, 1989).

Volcanic rocks (the Moat Volcanics), chiefly trachyte, tuff, breccia, and alkali rhyolite and comendite, are found in the eastern portion of the batholith. Only minor occurrences of such lithologies are present within the ring dikes of the western bathol ith. Several units of granite porphyry (grouped as the Mount Lafayette unit) occurring in the western batholith differ little in texture or mineralogy from the comendites of the eastern batholith. We show these rocks as volcanics (Figure 2) although definitive volcanic textures are generally lacking. Historically, the granite porphyry is treated as intrusive and not included within the Moat Volcanics (Williams and Billings, 1935; Eby and others, 1992).

Gabbro, diorite, and monzonite are present in the Mt. Tripyramid complex (Figure 2), a member of the younger White Mountain igneous province, that is spatially associated with the White Mountain batholith.

Figure 2. Geologic map of the White Mountain batholith (after Creasy, 1974; Osberg and others, 1978).

EMPLACEMENT OF THE WHITE MOUNTAIN BATHOLITH

Emplacement of the batholith occurred in middle and late Jurassic time, 201-155 Ma (Eby and others, 1992). The western half of the White Mountain batholith, exposed in the Franconia and Crawford Notch 15′ quadrangles, contains three igneous centers (W1-3 , Figure 2), the largest of which is 20 km in diameter. Igneous activity commenced in the western batholith with emplacement of the porphyritic quartz syenite (201 Ma and 193 Ma) and the quartz porphyry (195 Ma) of center W1 and the syenite and trachyte of center W2 (193 Ma). Subsequent intrusions of amphibole granite (187 Ma) [possible W3 (?)] and biotite granite (181 Ma) were widespread across the entire area of the batholith. Intrusion of peralkaline granite (177 Ma) in the eastern part of the pluton is considered an extension of the amphibole granite (Mount Osceola) event.

The eastern portion of the White Mountain batholith, exposed in the North Conway and Crawford Notch 15′ quadrangles, has at least four magmatic centers (Figure 3). Two centers with thick pyroclastic successions are interpreted as calderas (Noble and Bil lings, 1967; Fitzgerald, 1987; Fitzgerald and Creasy, 1988). Other centers where ring dikes or crescent- shaped intrusions are associated with epizonal plutons define more deeply eroded calderas. Caldera development here post-dates similar events in the western batholith by about 10-20 Ma. Dated units include the ring dike of center E2 (179 Ma); the Moat Mtn volcanic sequence (173-168 Ma) and ring dike (170 Ma) of center E3; and plutons of biotite granite (171 Ma and 155 Ma). We interpret the White Mo untain batholith as a sub-horizontal slice through a caldera field cut about 1.5 km thick and 1-2 km below the original landsurface. This excursion illustrates the field characteristics and structural relations of plutons, ring dikes, and volcanics that constitute the eastern half of the White Mountain batholith (Figure 3).

PLUTONS

Rocks forming plutons within the White Mountain batholith (Figures 2 and 3) are divided into two groups: (1) amphibole-bearing granites, quartz syenites and syenites and; (2) biotite granites.

Amphibole-bearing Granites, Quartz Syenites, and Syenites

Mount Osceola Granite. The Mt. Osceola Granite, a green amphibole +/- biotite granite, is the oldest member of the White Mountain magma series exposed in the North Conway quadrangle (Osberg and others, 1978; Eby and others, 1992). The number and original extent of plutons of the Mt. Osceola Granite within the North Conway quadrangle is not fully certain due to the complexity and abundance of younger rocks. A whole-rock Rb-Sr isochron for samples from both eastern and western portions of the batho lith yields an age of 186 m.y. (Eby and others, 1992) and indicates synchronous intrusion over a broad area. Ufliis age places the Mt. Osceola as the youngest member associated with the large magmatic center that forms the western portion of the ba tholith.]

The Mt. Osceola Granite is a medium- to coarse-grained hypersolvus granite that is dark green where fresh. It consists of an interlocking network of anhedral to subbedral microperthite 3-10 mm in diameter enclosing rounded grains of smokey quartz. Ferro hastingsite and locally annite are interstitial, late crystallized minerals. Fayalite (Fa95-99, Creasy, 1974) and sodic ferrohedenbergite (typical analysis Na3Ca40Fe56Mg1) are frequently present in accessory amounts and encased by reaction rims of ferroh astingsite. Characteristic accessories include allanite, sphene, zircon, fluorite, ilmenite, and monazite. Locally the Mt. Osceola is weakly peralkaline with ferrorichterite or riebeckite rimming ferrohastingsite. Miarolitic cavities may be locally abu ndant (one percent of outcrop area) and large (six to eight square centimeters). Pegmatite pods five to twenty centimeters across are abundant in many exposures of the Mt. Osceola Granite, locally forming up to two to three percent of the outcrop. Aplit e dikes, quartz veins, and fractures are abundant in all large exposures of the Mt. Osceola Granite. The aplitic dikes rarely exceed ten centimeters in width although they may be traced continuously for a hundred meters. Veins of quartz range from two t o five centimeters in width and commonly have open cores into which project well-formed crystals of quartz.

Figure 3. Geologic map of the eastern White Mountain batholith (Billings, 1928, modified by Osberg and others, 1978, and Creasy, 1986).

Protracted crystallization of feldspar progressively depleted the melt in Al and Ca and progressively enriched the melt in volatiles. Temperatures, total pressure, and water contents necessary for the stabilization of hydrous mafic phases were obtained a fter about 90% solidification. Compositions of interstitial amphiboles range from Al-poor ferrohastingsite –> ferrorichterite –> riebeckite. The occurrence of several amphiboles within samples deemed in hand specimen to be Mt. Osceola Granite suggest that the degree of fractionation of the crystallizing magma was locally variable. A fluid phase of sufficient volume to produce deuteric alteration and form pegmatitic pods was present during the final stages of crystallization.

Peralkaline Granite. Riebeckite-arfvedsonite granite forms an arcuate dike and small pluton intruding the Conway Granite of the Green Hills pluton. Petalkaline granites also form larger areas of outcrop in the eastern (e.g. on North Doublehead, Parnell, 1975) and central (e.g. Hart Ledge area, Henderson and others, 1977) batholith that appear to be young plutons spatially and genetically associated with Mt. Osceola Granite. Contacts between the peralkatine granites and the Mount Osceola are commonly gr adational.

The peralkaline granites are composed of subhedral grains of white microperthite (5-10 mm) and clear quartz (2-6 mm), blocky interstitial grains of riebeckite-arfvedsonite (<10 mm), and flakes and aggregates of interstitial biotite. Characteristic of this rock are abundant radiating arrays of golden colored astrophyllite. Fluorite, ilmenite, sphene, and apatite are common accessory minerals. Near contacts, miarolitic pods and cavities are developed on a cm-scale; here prismatic riebeckite crystals a re found up to 5 cm in length. One small body within the Hart Ledge complex (Wood, 1975) contains ferrorichterite (7%) in place of riebeckite-arfvedsonite; fayalite and ferrohedenbergite are accessories.

Alkali feldspar Quartz Syenite. Quartz syenite forms a small pluton within magmatic center E1 (Figure 2) and two arcuate bodies associated with the Hart Ledge complex of the central batholith (Figure 2; Henderson and others, 1977). The Hart Ledge complex is the youngest igneous activity in the central portion of the batholith, 169-162 Ma (Eby and others, 1992).

The quartz syenite is composed of tabular subhedral crystals of microperthite (14 mm) Anhedral quartz (<2 mm) is interstitial to and never included within these crystals of microperthite. Rounded grains of sodic ferrohedenbergite averaging 0.5 mm are present in all specimens though in variable amounts; commonly, these grains are enclosed within the microperthite. Ferrorichterite, the most abundant mafic mineral, forms interstitial grains and reaction rims on sodic ferrohedenbergite. That a vapor pha se may have formed is suggested by the occurrence of riebeckite. Rieheckite forms very thin rims on ferrorichterite, coats fractures within the ferrorichterite and penetrates pyroxene within. Further, tufts of acicular needles of riebeckite grown on a s ubstrate of ferrorichterite project into grains of quartz. These needles commonly less than one micron in diameter seems to dictate growth from a vapor phase. Ilmenite, allanite, zircon, and sphene are common accessories.

Alkali-feldspar Syenite. Syenite is an uncommon plutonic member of the White Mountain magma series; only two are described from the White Mountain batholith. The syenite occurring in the central portion of the batholith (Figure 2; W ood, 1977; Henderson and others, 1977)) ispart of the Hart Ledge complex. The syenite is a coarse rock, dark green where fresh; blocky microperthite and ferrohastingsite (about 10%) account for ninety-five percent of the hand specimen; fayalite and ferroh edenbergite are minor accessories. The syenite contains miarolitic pods of coarse prismatic ferrohastingsite and irregularly-shaped quartz.

The syenite is of interest because of the spatial and genetic relationship to the widespread Mt. Osceola Granite and to the peralkaline granites and quartz syenite. Significantly, REE data for the Hart Ledge complex (Creasy and others, 1979; Eby and othe rs, 1992) show a positive europium anomaly for the syenite, but substantial negative anomalies for the quartz syenites and peralkaline granites. The syenite may represent the cumulus feldspar and the peralkaline granites may represent residual liquids de rived from the crystallization of a Mount Osceola-type parental magma. No other analyzed rocks from the batholith show negative Europium anomalies.

The variation in composition of these amphiboles, consistent with principles of crystal fractionation, suggests the riebeckite granite is more strongly fractionated than the other two units. The Mt. Osceola Granite, the quartz syenite of Mt. Tremont, and the riebeckite granite, respectively, were derived from the same or similar magmas that had undergone increasingly greater degrees of CryStal fractionation. The apparent variation in degree of fractionation may reflect the sequential evolution of a sing le parent magma or may result from the exposure, at current levels of erosion, similar magmas that had fractionated to differing degrees.

Biotite Granites

Conway Granite. This pink sub-alum inous biotite granite is the most extensive unit in the North Conway quadrangle. Billings (1928) showed the Conway Granite as a single irregularly shaped pluton and as the youngest otthe White Moun tain magma series. More detailed mapping (Osberg and others, 1978) has recognized several distinct plutons of biotite granite on the basis of texture and outcrop geometry (Figure 3). Absolute ages for plutons in the North Conway quadrangle are within th e range of 183-155 Ma (Eby and others, 1992). Field relations suggest that emplacement of these plutons was not synchronous across the quadrangle but related to individual magmatic centers. The Birch Hill pluton (Osberg and others, 1978) is the largest pluton. The Conway Granite of this pluton becomes finer grained, porphyri tic, and miarolitic where it intrudes the Moat Volcanics. The Gardiner Brook pluton (Osberg and others, 1978) intrudes Moat Volcanics on Mount Kearsarge and is associated with the magmatic center defined by ring IV (Figure 3). The Conway Granite of this pluton shatters Silurian metasedimentary rocks (Hatch and others, 1984) along the East Branch of the Saco River. A third pluton underlies most the Green Hills, the prominent nort h-south oriented ridge forming the east side of Mt. Washington Valley; this pluton is well exposed on Black Cap mountain.

The Conway Granite is a medium- to coarse-grained pink biotite two-feldspar granite. Values of microperthite:oligoclase range from 2:1 to 10:1 and average 4-5:1 (Creasy, 1974). Annite (Ann90) forms anhedral interstitial grains up to 5 mm in size. In co ntrast with other members of the White Mountain magma series, fayalite and ferrohedenbergite are absent. Subordinate amphibole is present in some samples. Zircon, allanite, apatite, sphene, and fluorite are common accessories. Near intrusive contacts, the Conway Granite shows a variety of textures that may grade into each other on the outcrop scale: porphyritic, aplitic, miarolitic, and pegmatitic. Miarolitic cavities are typically of mm-scale and bounded by euhedral crystals of quartz and feldspar. A zone of miarolitic cavities ranging up to several meters is present within the Conway Granite adjacent to the Moat volcanics on the east side of the Moat Range. This and similar occurrences of miarolitic cavities in the eastern batholith have produced many beautiful smokey quartz crystals. Weakly developed banding on the cm- to dm-scale resulting from variations in grain size and/or mineral concentrations is developed near some contacts. Lithic fragments of any type are sparse in the Conway Granite.< p> Black Cap Granite. The Black Cap Granite (Billings, 1928) is a fine-grained pink aplitic biotite granite that outcrops in two small areas in the North Conway quadrangle. It is composed of quartz, microperthite, subordinate oligiocla se, and chloritized biotite. Accessories include zircon, magnetite, apatite, and fluorite. The Black Cap Granite is shattered and intruded by the Conway Granite (Green Hills pluton) on the flanks of Black Cap. Billings considered this rock an early lit hologically distinct ‘phase’ of the Conway Granite. Osberg and others (1978) suggest that the Black Cap granite to be coeval with and a roof facies of the Conway Granite.

RING DIKES

Ring dikes of the eastern White Mountain batholith consist of porphyritic quartz syenite and subordinate porphyritic syenite. The porphyritic quartz syenite is similar in appearance across the batholith but occurrences in the eastern and western parts of the batholith are distinguished as the Albany and the Mount Garfield, respectively. Ring dikes define at least four magmatic centers in the eastern batholith (Figure 3). Ring dikes of centers E2 and E3 yield Rb/Sr ages of 179 and 170 Ma, respectively ( Eby and others, 1992). These ring dikes are outwardly dipping at 40-80 degrees with well developed chill margins adjacent to older rocks. The ring dikes are not seen to cut Table 1. Modes from the eastern portion of the White Mountain batholith (Billings, 1928; Osberg and others, 1978; and Davie, 1975).

Stop No.
Quartz
Alkali feldspar
Plagioclase
Biotite
Amphibole
Ferrohedenbergite
Fayalite
Accessories
#la
35
49
14
2
0
0
0
tr
#lb
22
72
3
tr
1
tr
2
tr
#2a
10
79
0
1
8
8
1
3
#2b
16
70
0
0
9
2
tr
3
#2c
20
68
0
tr

1
tr
3

Stop No.
Quartz
Alkali feldspar
Plagioclase
Biotite
Amphibole
Ferrohedenbergite
Fayalite
Accessories
#5
13
74
0
1
12
0
0
tr
#6
29
48
17
6
0
0
0
tr
#7
33
49
16
2
0
0
0
tr
#8
39
54
0
1
5
0
0
1
#9
28
47
18
7
0
0
0
1
la
lb
2a,b,c
5
6
7
8
9
Conway Granite, Birch Hill pluton, Hurricane Mtn Road.
Mt. Osceola Granite, Rattlesnake Mtn, Redstone area.
Albany Porphyritic Quartz Syenite, three distinct types within ring dike E3, Little Attitash Mtn.
Albany Porphyritic Quartz Syenite, Jackson Falls.
Black Cap Granite, Thorn Mtn.
Conway Granite, Gardiner Brook pluton, Burnt Knoll Brook.
rieheckite granite, North Doublehead.
Conway Granite. Green Hills pluton, Black Cap mountain.

each other but relationships to other units indicate their emplacement was not simultaneous. Although similar in mineralogy, the ring dikes of different centers are distinctive in mineral chemistry and in texture.

In fine structure individual ring dikes are themselves multiple intrusions. For example, at least four separate intrusions of porphyritic quartz syenite and syenite occur in ring E3 (Table 1; Figure 3). The increasing abundance of feldspar phenocrysts a nd of total quartz content with decreasing age suggests (Davie, 1975) successive differentiates of a subjacent magma body. Inclusions present in ring BI document a similar structural relationship.

Albany Porphyritic Quartz Syenite

The Albany Porphyritic Quartz Syenite contains phenocrysts of microperthite (5-10 mm) and subordinate quartz (24 mm); minor phenorryst phases include ferrohedenbergite (Ca45Fe45-40Mg10-15), fayalite (Fa90-94) and ilmenite (Creasy, 1974). Variation of tot al phenocryst abundance (10-60%) and phenocrystic feldspar:quartz (5-10:1) is noted both within a ring dike (e.g. #2a, b, c of Table 1; Davie, 1975) and among different ring dikes. Quartz phenocrysts are subangular in chilled border zones but are rounded with seriate margins in coarser varieties.

The groundmass is uniform in grain size (<2-3 mm) within an intrusion (except near contacts) but shows variation among different intrusions. Minerals of the groundmass are anhedral quartz, alkali feldspar, subordinate oligioclase, and minor ferrohasti ng site and annite. Ferrohastingsite occurs as anhedral interstitial grains and as rims on the ferrohedenbergite and fayalite. In both occurrences it may poildiltically enclose small quartz grains. Grunerite is found as reaction rims on fayalite and is surrounded by ferrohastingsite. Accessories include allanite, sphene, zircon, and fluorite. Secondary sericite, biotite and chlorite are commonly present.

The mineralogic transition from the anhydrous phenocryst (intratelluric) assemblage to the hydrous groundmass (emplacement) assemblage is written as a simplified end-member reaction (Creasy, 1974):

Fe2SiO4 + 2CaFeSi2O6 + FeTiO3 + [NaA1Si3O8 + H20] –> NaCa2Fe(2+)Fe(3+)Si6(Al,Ti)2O22(OH)2 + [2SiO2]
fayalite + 2 ferrohedenbergite + ilmenite + albite = ftrrohastingsite + quartz,

where [ ] indicates probable melt components. The stabilization of the hydrous phase, ferrohastingsite, relative to layalite and ferrohedenbergite requires a reduction in temperature or a combined reduction in temperature and total wessure. The textural and physicochemical changes coincide with emplacement of the ring dikes.

Poryphyritic Syenite

The porphyritic syenite is the oldest and least evolved lithology present within ring dikes. In contrast to the yritic quartz syenite, total phenocryst abundance is commonly less than fifteen percent, reaction rims of ferrohastingsite are minor or lacking , and alkali feldspar lacks exsoluflon textures. A two-pyroxene assemblage for the ferrohedenbergite typical of the Albany:

ferropigeonite (Ca8Fe70-82Mg22) + ferroaugite (Ca40-37Fe48-48Mg25-18) –>
[porphyritic syenite]

ferrohedenbergite + fayalite + quartz. [porphyrific quartz syenite]

This reaction marks the boundary of the pyroxene ‘forbidden zone’ and bulk grain compositions cited above yield T = 825 degrees C (Lindsley, 1983). Compositions of orthopyroxene host and ferroaugite lamellae of the inverted ferropigeonite give temperatur es of about 700-750 degrees C.

VOLCANIC ROCKS

The Moat Volcanics (Billings, 1928) are exposed in the Moat Range to the west of North Conway (E3, Figure 3; Figure 4); a second major occurrence is on Kearsarge North (E4, Figure 3). The Moat Range sequence shows pronounced and laterally persistent laye ring (Billings, 1928; Noble and Billings, 1967) striking northwest and dipping 30-40 degrees to the northeast. Billings (1928) interpreted these exposures as an originally flat-lying sequence that subsided and rotated along a ring fracture (now the ring dike). Noble and Billings (1967) suggest syn- or post-subsidence accumulation of the sequence within a caldera. An intra- caldera setting is supported by the detailed mapping of Fitzgerald (1987; Figure 4). The original extent of the volcanics outside the caldera is a major question. The isotopic systematics (Eby and others, 1992) indicate that fractional crystallization of quartz syenitic magma with minor amounts of crustal contamination can produce the comendites of the Moat Range sequence. Five cr ystal-rich units dominate the 3.6 km of volcanic stratigraphy of the southern Moat Range (Fitzgerald and Creasy, 1988; Figure 4). These are (base to top): lower comendite (380 m), feldspathic welded tuff (425 m), the R & B comendite (1070 m), trachyt e and trachyte breccia (300 m), and Red Ridge comendite (>1050 m). The lower oimendite contains interbeds of polymict lapilli tuff and breccia that are thickest and coarsest adjacent to the enclosing ring dike. Internal structures (fiamme, oriented l ithic clasts) developed within the lower two units but are lacking in the upper comendites. Several thin but persistent horizons of bedded ash tuff, lapilli tuff, and mesobreccia and megabreccia lie above and below the trachyte. Bedding generally parall els the surrounding ring dike (E3) and dips radially inward at 20-40 degrees.

Figure 4. Geologic map of the Moat Range (Billings, 1928; Fitzgerald, 1987).

The northern Moat Range is dominated by the upper comendite units which are generally homogeneous but show variation of abundance and proportion of phenocrysts and lithic fragments. Bedded tuffs here are quite similar to the comendites but exhibit beddin g (averaging 2 m), eutaxitic textures, and are more lithic-rich. A section of chaotic megabreccia (60 m thick, clasts up to 4 m) within the bedded tuffs is interpreted as a lahar or debris flow (Fitzgerald, 1987). A polymict clast-supported mesobreccia is present on the summit of North Moat Mtn (3196′), highest point of the Moat Range.

Keaasarge North (E4, Figure 3) exposes a mix of comendite, feldspathic tuff, and meso- and megabreccias (Billings, 1928) similar to those found on North Moat. The comendites lie topographically below the more extensive breccias and are lithic-rich. As i n the Moat Range sequence, the breccias on Kearsarge North are polymict (although in any horizon one or another clast lithology may predominate) and the matrix: clast ratio varies greatly.

The comendites are blue-gray to pink rocks contain variably abundant phenocrysts (1-3 mm) of quartz and microperthite and rare phenocrysts of biotite, ferrohastingsite, ferrohedenbergite, and riebeckite set in a matrix of quartz and alkali feldspar. Acce ssories include apatite, fluorite, zircon, and magnetite. Lithic fragments (mm- and cm-scale) constitute 1-5% of most comendite samples. Lithic types include hornfels, cogenetic volcanic rocks, porphyritic quartz syenite, and rarely cogenetic plutonic r ocks. The trachyte consists of phenocrysts (2-3 mm) of pink alkali feldspar set in a dense (<0.1 mm) groundmass of alkali feldspar. Accessories include abundant hematite and minor zircon, magnetite, epidote and clinozoisite. The breccias contains an gular to subrounded rocks ranging from a few centimeters to a meter in size. Lithic fragments include a variety of metamorphic rock lithologies, Paleozoic intrusive rocks, and cogenetic volcanic and hypabyssal rocks.