Jurassic phase, Alaska-Aleutian Range batholith, undifferentiated

Unit symbol: Jgr
Age range Jurassic (182.7 to 152.1 Ma)
Lithology: Igneous - Plutonic
Group name: Plutonic rocks
The Alaska-Aleutian Range batholith (Reed and Lanphere, 1972) ranges from granite and granodiorite to quartz diorite and tonalite and includes some quartz monzonite. As defined, the batholith includes Jurassic (included here and in unit Jtr), Cretaceous (included in unit Kgu), and Tertiary (included in units Tpgr and TKgd) phases. The Jurassic phase, in general, is exposed on the east side of the batholith. A subunit of the Jurassic phase, trondhjemite is described in unit Jtr. The most extensive area of exposure is in the Aleutian Range on the west side of Cook Inlet, where the plutons lie to the southeast of the Castle Mountain and Lake Clark Fault Systems and are bounded on the east by the Bruin Bay Fault. The Aleutian Range exposures extend to the southwest a distance of 430 km to Becharof Lake and, on the basis of geophysical trends, have been inferred to continue in the subsurface along the Alaska Peninsula (Case and others, 1981, 1988) and possibly into the Bering Sea. The unit is also exposed in the Talkeetna Mountains and Anchorage quadrangles. The rocks there are truncated on the south by the Border Ranges Fault System. Generally thought to be the plutonic equivalent of volcanic rocks of Talkeetna Formation (unit Jtk, here), northern exposures of the plutons are commonly found in close proximity to the Talkeetna Formation. The Alaska-Aleutian Range batholith has been best described and dated by Reed and Lanphere (1969, 1972, 1973). Locally subdivided by lithology, significant phases are granite, granodiorite, quartz monzonite, quartz diorite, tonalite, and gabbro. The quartz diorite and tonalite phases form the majority of the batholith on the Alaska Peninsula and form the southwest part of the Talkeetna Mountains section. These rocks are typically medium-grained, gray equigranular rocks, and are locally foliated. Either clinopyroxene or hornblende is the dominant mafic mineral. Biotite increases in proportion to the presence of quartz and potassium feldspar. Detterman and Reed (1980) reported that rocks of unit in Iliamna region grade to diorite but they did not observe it grading into quartz monzonite or granodiorite (their unit Jqm). Inclusions of fine-grained mafic rocks or of porphyro-aphanitic volcanic rocks are common. K/Ar and 40Ar/39Ar ages generally range from 174.7±8.8 to 146±4.3 Ma (Reed and Lanphere, 1973; Magoon and others, 1976; Csejtey and others, 1978; Hacker and others, 2011); a number of samples yielded younger ages that were considered suspect or thought to be reset by younger plutonism; the majority of concordant ages are between about 165 and 160 Ma. U/Pb ages are bimodal—on the Alaska Peninsula they range between 183 and 164 Ma and no inheritance is indicated, whereas in the Talkeetna Mountains the ages range from as old as 208 Ma to 187 Ma, and most sample analyses indicate inheritance (Rioux and others, 2007). A diorite pluton has been mapped on the eastern side of the batholith in northeast Iliamna quadrangle and it mostly likely continues in the adjacent Lake Clark quadrangle (Detterman and Reed, 1980). Unit also includes a group of lamprophyre and basalt dikes on west side of Cook Inlet that intrude the quartz diorite. Two rocks sampled northeast of Lake Iliamna yielded mid-Cretaceous K/Ar age determinations (Reed and Lanphere, 1972) and may indicate presence of an otherwise unrecognized mid-Cretaceous magmatic event in the batholith or may more likely indicate resetting by nearby middle Tertiary plutons. Granodiorite and quartz monzonite phase is also widely distributed, occurring throughout the range of exposure. On the Alaska Peninsula, it is exposed on the eastern side of the batholith; mapping in the northern section of the batholith does not separate this phase from the quartz diorite and tonalite phase. The granodiorite is typically whitish-gray, medium-grained and biotite-bearing with minor hornblende and accessory primary muscovite, and the quartz monzonite is medium-grained, light-gray with a pinkish cast. K/Ar age determinations from both sections range from 180.4±5.0 to 140.0±4.2 Ma (Csejtey and others, 1978; Shew and Lanphere, 1992; Wilson and Shew, 1992), but the majority of ages and the most reliable ones are older than 157 Ma, U/Pb zircon ages range between 177 and 169 Ma, all from the northern section of the batholith (Rioux and others, 2007). Granite is the least common phase and is typically found at the southern end of the exposure range; it is typically light-gray, porphyritic, contains more biotite than hornblende, and is locally foliated (Riehle and others, 1993). K/Ar ages range between 173.6±2.0 and 159.6±2.0 Ma (Shew and Lanphere, 1992); no U/Pb ages have been determined on this granitic phase. Detterman and Reed (1980) and Riehle and others (1993) also reported small bodies of dark gray, diabase and gabbro, which are included here. Trondhjemite, a subunit of the Jurassic phase, is described in unit Jtr

Source map information

Source map Detterman, R.L., and Reed, B.L., 1980, Stratigraphy, structure, and economic geology of the Iliamna quadrangle, Alaska: U.S. Geological Survey Bulletin 1368-B, 86 p., 1 sheet, scale 1:250,000.
Symbol Jm
Unit name Mafic and ultramafic plutonic rocks
Description Gabbro, hornblende gabbro, hornblendite, and pyroxenite
Lithology Igneous

Correlated geologic units

Label Jgaa
Description Largely gabbroic rocks, may include minor ultramafic or dioritic rocks, associated with Alaska-Aleutian Range batholith
Geologic age Toarcian to Kimmeridgian
Geologic setting Intrusive, gabbro
Lithology Form Importance
Gabbro < Gabbroic < Plutonic < Igneous Pluton Major
Hornblendite < Ultramafic < Plutonic < Igneous Pluton Minor
Pyroxenite < Ultramafic < Plutonic < Igneous Pluton Minor