U.S. DEPARTMENT OF THE INTERIOR GEOLOGIC INVESTIGATIONS SERIES U.S. GEOLOGICAL SURVEY I–2697 Qtl Ta Ktl Ta CORRELATION OF MAP UNITS microcline phenocrysts as much as 10 cm long making up 2 to 10 The pre-Tertiary rocks of the Tower Peak quadrangle fall into two groups: (1) an here has burst its shell of more mafic differentiates and intrudes older granitic and Qng Qtl Qg a c percent of the rock. Quartz typically occurs in 1/2-cm clots. Mafic L-shaped area of older plutonic and metamorphic rocks, 3 to 10 km wide, that metamorphic rocks, and (2) the Half Dome Granodiorite, which is interpreted as an Qal Qng Tvs Qg 70 Tr p SURFICIAL DEPOSITS Qtl 75 c Qtl Ktl Qga mineral content about 10 percent. Asymmetric schlieren locally extends diagonally both northeast and southeast from near the center of the quadran- earlier, more mafic differentiate of the same magmatic pulse as that for the Ta c Kfl Qal Qtl Qls Qng Holocene Ktl Qtl abundant gle; and (2) a younger group of large, probably composite intrusions that cover large Cathedral Peak Granodiorite. Qtl Ta 70 QUATERNARY a Qal c areas in adjacent quadrangles and extend into the Tower Peak quadrangle from the The northern composite intrusion of the younger group includes the Qal c Kfl Granodiorite of Fremont Lake (Cretaceous)—Medium-dark-gray, Qng Tvs Qg Late Qtl Qtl 65 Tr p Qg medium-grained, hornblende-biotite granodiorite. Where horn- east, north, and southwest. granodiorite of Topaz Lake, a rock almost identical in appearance and age to the Qtl Qtl Pleistocene Tr p Qng Qng 70 a Cathedral Peak Granodiorite, but separated from the latter by a 6- to 7-km septum of Ktl Ta Qg VOLCANIC ROCKS blende-rich, contains abundant lenticular or elongate mafic inclu- Qng Qtl Qtl 70 65 OLDER PLUTONIC AND METAMORPHIC ROCKS older rocks. The Topaz Lake pluton is bordered on the south by the granodiorite of c sions, commonly uniformly distributed, but locally in clusters. In the a a a Qal a Qg 60 Ta southwest part of its occurrence in this quadrangle, it is largely The L-shaped remnant of older plutonic and metamorphic rocks that extends Fremont Lake. The Fremont Lake pluton appears to bear the same relationship to the Tvs 55 c 70 75 northeast and southeast from the center of the quadrangle contains ten plutons with granodiorite of Topaz Lake that the Half Dome Granodiorite does to the Cathedral Kfl a Tr p Miocene mafic free, lacking in inclusions, and tending toward granitic com- Tr p contiguous areas of at least 2.5 sq. km and several complexes of dikes and small Peak Granodiorite. Qal Qtl 65 a a TERTIARY position. Map unit is named for exposures near Fremont Lake where c Qal Qng Ta Tvs Tbb irregular masses. These plutons and dikes intruded silicic and mafic metavolcanic The southwestern composite intrusion of the younger group includes four large a it was mapped by Giusso (1981) in the Sonora Pass quadrangle to Qng 35 c 65 a 50 Kfl a Qal 65 rocks and miogeoclinal metasedimentary rocks. Mutual age relations between the plutons in the southwest corner of the Tower Peak quadrangle and in adjacent Tvs 55 Qng Miocene to the north. The unit extends west into the Pinecrest quadrangle where Ktl Qtl Qal 50 65 a a65 Tvs plutonic rocks are incompletely determined, either because some of the rocks are not a 60 a a Oligocene it has been called the granodiorite of Kinney Lakes (Huber, 1983) quadrangles. These plutons show some evidence of being consanguineous, yet other Tbb Qtl a 80 a in geographically contiguous sequences or because the contacts have ambiguous age evidence suggests a significant hiatus in the sequence. The complex includes, from Qtl Tbb Qg Klc Granodiorite of Long Canyon (Cretaceous)—Dark hornblende-biotite Qal aa85 75 Klc mvr PLUTONIC ROCKS criteria. Structures in this remnant are generally steep, trend northwesterly, and are oldest to youngest, the quartz diorite of Mt. Gibson, the granodiorite of Bearup Tr p 70 [Lines and arrows indicate intrusive relations] granodiorite and quartz-diorite with mafic mineral content of about c c a c cut off sharply by the younger plutons. Lineations, foliations, and zones of recrystal- Lake, the granodiorite of Lake Vernon, and the granodiorite of Boundary Lake. A Tvs a c Qg South and 30 percent and with abundant mafic inclusions aa70 80 lization and shearing, all of supposed metamorphic or tectonic origin, are present in swarm of fine-grained diorite dikes cuts the Mt. Gibson and Bearup Lake, but is 75 msm msq southwestern Northern Eastern Qal 70 gb a75 Klc a 80 a Eastern area some plutons and suggest at least one period of dynamic metamorphism during or apparently cut off by the granodiorite of Lake Vernon. This succession of events Tr p 70 a Qtl area area area a apa a a 75 c Qal 80 70 a Qg Tr p Kcp Cathedral Peak Granodiorite (Cretaceous)—Medium- to coarse- after the period of emplacement of these plutons. The metamorphic and early mafic suggests a time break and interval of cooling. However, the granodiorite of Bearup msh 60 a70 80 Kbl Ktl Kcp rocks are described first below, followed by the plutonic rocks of the northeast arm Lake grades in color index from very near that of the Mt. Gibson where it is in Qg mvr c c grained, hornblende-biotite granodiorite that contains conspicuous 70 a70 85 of the "L", and then the rocks of the southeast arm. Klh Qg a 85 Klv Kfl Khd blocky phenocrysts of microcline as much as 10 cm long. The intrusive contact with the Mt. Gibson to very near that of the Lake Vernon where the a Metamorphosed sedimentary rocks appear to be the oldest rocks. They include latter intrudes the Bearup Lake. This suggests a genetic relationship extending a a aa phenocrysts commonly occur in aligned swarms and asymmetric 80 mvb Qal 75 Kbu Klc Kgm Ktl di ap schlieren are locally abundant pure white orthoquartzite, in beds a few meters thick, that display thin partings bear- across the time gap. 80 80 Kmgc a a mvr i Kut Khd Half Dome Granodiorite (Cretaceous)—Medium- to coarse-grained, ing andalusite and biotite and common internal crossbedding. These are interbedded 80 Tvs a Kmgf CRETACEOUS c 80 a DIORITE DIKES Qtl a 85 equigranular hornblende-biotite granodiorite with abundant elongate with thick (10 to 50 m) units of biotite-andalusite-sillimanite(?) schist, coarsely c Kdl 80 Qg c Kpm Kt Kmp Kbp c a a 75 crystalline marble, and fine to coarse beds of pale-green, calc-silicate hornfels. Sets of diorite dikes (not shown on the geologic map) with at least three 65 c a or flattened mafic inclusions. Characterized by euhedral hornblende 80 c 80 a These metasediments are complexly deformed. The southern body around Benson orientations intrude the granite of Avonelle Lake, and at least one set also intrudes a c g Klh Kal ap Kdl prisms as much as 1 cm long, biotite books as much as 1 cm across, Kfl 85 Lake is mainly quartzite, with some thick biotite-andalusite schist bodies near the the quartz diorite of Mt. Gibson and granodiorite of Bearup Lake. These dikes are a a c and conspicuous sphene. Near Tilden Lake it is mafic-poor and has 80 c Klh c msh a top and marble at the base, folded into a large anticline with a horizontal crest and confined to the southwest part of the quadrangle and do not appear to occur in the 80 i Klh large microcline phenocrysts Qal c 80 vertical limbs. The minimum stratigraphic thickness exposed in this body is 750 m. northeastern part of the Avonelle Lake pluton. One set trends due north and is about 75 c 75 ap c CRETACEOUS Kgm Alaskite of Grace Meadow (Cretaceous)—Sugary, medium- to fine- di gb The northern body, around Bigelow and Snow Lakes, contains abundant marble bod- vertical, another set trends northwest and is about vertical, and the third strikes a a msq Qtl c OR OLDER grained, light-pink to white alaskite with rare dark minerals. ies and about equal amounts of quartzite, biotite schist, and calc-silicate hornfels. It roughly east-west and dips 30 to 45 north. The first two sets appear to predate the 80 ° ° a c Contains abundant inclusions of diorite, gabbro, quartzite, biotite Qal METAMORPHIC ROCKS has been folded at least twice, the first time isoclinally. The latest deformation was formation of the Mahan Peak complex and the intrusion of the Mt. Gibson pluton c schist, and marble a a Qng into a flat-topped dome with nearly vertical flanks along the intrusive contacts. because they are found as inclusions in the former. The diorite dikes probably c CRETACEOUS c a Kut 80 c i mvb mvr Granite of Upper Twin Lake (Cretaceous)—Coarse, equigranular, a Small bodies and inclusions of metasedimentary rocks in several of the plutons indi- represent periods of crustal extension, although it is not clear how extension alone 70 di ms msq msm OR OLDER a 60 Qg 85 pink biotite granite. Locally has a very strong vertical lineation, a Qal Kcp cate that this terrane was once more widespread. In the extreme northeast part of the would produce a dike-swarm dipping 30 north. Qng ° 80 45 Kbp msq probably of metamorphic origin 75 Qg quadrangle, metasedimentary rocks, which may be in depositional sequence with the Where the diorite dikes intrude granite, they are bordered by thin, discontinuous 60 g a g Kfl 70 a Kbp Granite of Bond Pass (Cretaceous)—Gray, moderately foliated, a msq 60 g ms msm metavolcanic rocks, include quartzite and stretched conglomerate.