Orthogneisses of granite, quartz monzonite, monzonite, and tonalite, collectively termed granitoid, occur locally as isolated enclaves within the Early Cretaceous granulite terrain (Western Fiordland Orthogneiss—WFO) in Fiordland, southwest New Zealand. Discordant U‐Pb zircon isotopic data (7 fractions) from four granitoid samples from enclaves at George Sound, define an upper intercept age of 341 ± 34 Ma that is interpreted as approximating the time of formation of the granitoid suite. The lower intercept age of 93 ± 37 Ma is interpreted as approximating the time of zircon isotopic disturbance by major episodic Pb loss. Isotopic disturbance probably accompanied high‐temperature conditions at the time of incorporation of the granitoid rocks into the WFO protolith, and during subsequent granulite meta‐morphism; both occurred in the interval c. 120–110 Ma. Five whole‐rock samples from enclaves at George Sound yield an apparent Rb‐Sr isochron age of 391 ± 48 Ma, in agreement with the zircon data, and a 87Sr/86Sr initial ratio of 0.7044 ± 10. The low 87Sr/86Sr initial ratio indicates that these mid‐Paleozoic granitoids were derived from an isotopically primitive source, distinct from the mature source typical of mid‐Paleozoic Karamea Suite granitoids abundant elsewhere in western New Zealand. The granitoid enclaves within WFO show influences of several different sources: most of the rocks have A‐type compositions suggestive of high‐temperature crustal anatexis; others have compositions corresponding to normal calcalkaline and highly fractionated varieties thereof. One granitoid rock has a Sr isotopic composition and inherited zircon component compatible with melting of a Paleozoic sedimentary source; others have Sr isotopic compositions compatible with Early Cretaceous mixing of this crustal component with a mantle‐derived mafic component. The granitoids provide evidence linking WFO to a mid‐Paleozoic country rock similar to the central Fiordland metasediments.

Two distinct types of compositional zoning are recognised in idioblastic to subidioblastic garnets in amphi‐bolite facies metapelites in western Fiordland. One type occurs in kyanite (± sillimanite)‐bearing rocks and is characterised by a core to rim increase in CaO content, and a sympathetic decrease in FeO and MgO, with MnO remaining essentially constant, or decreasing slightly near the rim. This zoning pattern formed during garnet growth, and records adjustment of mineral equilibria under conditions of increasing load pressure. Garnets showing the same zoning pattern occur within metapelitic rocks in rafts within nearby Western Fiordland Orthogneiss (equivalent to western Fiordland granulites), and within a contamination zone overprinting this orthogneiss at the margin of a highly aluminous raft. Garnet zoning thus provides evidence that these rocks experienced the same Early Cretaceous tectono‐metamorphic event. The pressure increase has been attributed to crustal thickening by tectonic loading in a plate boundary collisional regime. A much less common type of zoned garnet occurs in sillimanite + K‐feldspar‐bearing metapelites and is characterised by a core to rim decrease in CaO and MgO contents and a sympathetic increase in FeO and MnO near the rim. This zoning pattern is interpreted to result from post‐growth volume diffusion, and records metamorphism at significantly lower pressures than that indicated by rims of the growth‐zoned garnets described above. Overprinting relationships suggest that the diffusion‐zoned garnets formed prior to crystallisation of the growth‐zoned garnets. At least some of the diffusion‐zoned garnets are products of Early Cretaceous metamorphism, and, together with the growth‐zoned garnet, indicate two distinct episodes of garnet growth during this period. Isotopic evidence presented recently for mid‐Paleozoic metamorphism in western Fiordland suggests that separate metamorphic terranes of Early Cretaceous and mid‐Paleozoic age are probably present in this area.

The Idono Complex of west-central Alaska is a fault-bounded, approximately 300 km2 fragment of Early Proterozoic continental crust surrounded by overlap assemblages and younger terranes accreted in Mesozoic time. It is composed of granitic to dioritic orthogneiss, amphibolite, and metasedimentary rocks. Trace element compositions of the granitoids and amphibolite suggest rock formation in a subduction-related volcanic arc terrain. Nine zircon fractions from three samples of granitoid orthogneiss define a U-Pb discordia line intersecting concordia at 2062 +/- 7 Ma and 182 +/- Ma. The upper and lower intercepts are interpreted, respectively, as approximations of the time of granitoid crystallization and major episodic Pb-loss. The lower intercept age is similar to that indicated by some hornblende and biotite K-Ar ages. Other biotite, hornblende, and white mica K-Ar ages record later isotopic disturbance in Early Cretaceous time. Nd isotopic compositions at 2.06 Ga for tonalite orthogneiss (epsilon-Nd(t) = +2.8) and amphibolite (epsilon-Nd(t) = +1.8 and +2.3) indicate that Early Proterozoic crust formation in the Idono Complex involved significant additions of mantle-derived magma. These rocks yield T(Sm-Nd) (depleted mantle) model ages of 2.0 to 2.1 Ga, similar to the U-Pb zircon upper intercept age. Involvement of Archean crust in the formation of at least some rocks of the Idono Complex is indicated by a granitic orthogneiss (epsilon-Nd(t) = -4.6), which yields a T(Sm-Nd) model age of 2.5 Ga. The approximately 2.06-Ga age of the Idono Complex is similar to that indicated for provenance(s) of widely distributed sedimentary rocks in western North America and may represent a fragment of the North American craton displaced by northward movement of the Pacific plate. At such, rocks of the Idono Complex may provide important insights into both Early Proterozoic evolution along the craton margin, and subsequent displacements.

Research Article| December 01, 1990 Kilbuck terrane: Oldest known rocks in Alaska Stephen E. Box; Stephen E. Box 1U.S. Geological Survey, U.S. Courthouse, Room 656, Spokane, Washington 99201 Search for other works by this author on: GSW Google Scholar Elizabeth J. Moll-Stalcup; Elizabeth J. Moll-Stalcup 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Joseph L. Wooden; Joseph L. Wooden 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar John Y. Bradshaw John Y. Bradshaw 3U.S. Geological Survey, 4200 University Drive, Anchorage, Alaska 99508 Search for other works by this author on: GSW Google Scholar Author and Article Information Stephen E. Box 1U.S. Geological Survey, U.S. Courthouse, Room 656, Spokane, Washington 99201 Elizabeth J. Moll-Stalcup 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Joseph L. Wooden 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 John Y. Bradshaw 3U.S. Geological Survey, 4200 University Drive, Anchorage, Alaska 99508 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1990) 18 (12): 1219–1222. https://doi.org/10.1130/0091-7613(1990)018<1219:KTOKRI>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Stephen E. Box, Elizabeth J. Moll-Stalcup, Joseph L. Wooden, John Y. Bradshaw; Kilbuck terrane: Oldest known rocks in Alaska. Geology 1990;; 18 (12): 1219–1222. doi: https://doi.org/10.1130/0091-7613(1990)018<1219:KTOKRI>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Kilbuck terrane in southwestern Alaska is a narrow, thin crustal sliver or flake of amphibolite facies orthogneiss. The igneous protolith of this gneiss was a suite of subduction-related platonic rocks. U-Pb data on zircons from trondhjemitic and granitic samples yield upper-intercept (igneous) ages of 2070 ±16 and 2040 ±74 Ma, respectively. Nd isotope data from these rocks suggest that a diorite-tonalite-trondhjemite suite (εNd [T] = +2.1 to +2.7; T is time of crystallization) evolved from partial melts of depleted mantle with no discernible contamination by older crust, whereas a coeval granitic pluton (εNd [T] = -5.7) contains a significant component derived from Archean crust. Orthogneisses with similar age and Nd isotope characteristics are found in the Idono complex 250 km to the north. Early Proterozoic rocks are unknown elsewhere in Alaska. However, Phanerozoic plutons cutting several "continental" terranes in Alaska (southern Brooks Range and Ruby, Seward, and Yukon-Tanana terranes) have Nd isotope compositions indicative of Early Proterozoic (or older) crustal components that could be correlative with rocks of the Kilbuck terrane. Rocks with similar igneous ages in cratonal North America are rare, and those few that are known have Nd isotope compositions distinct from those of the Kilbuck terrane. Conversely, provinces with Nd model ages off 2.0-2.1 Ga are characterized by extensive 1.8 Ga or younger plutonism, which is unknown in the Kilbuck terrane. At present the case for a North American parentage of the Kilbuck terrane is not compelling. The possibility that the Kilbuck terrane was displaced from provinces off similar age in other cratons (e.g., Australian, Baltic, Guiana, and west African shields), or from the poorly dated Siberian craton, cannot be excluded. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A c. 700 km2 area of northern Fiordland (South Island, New Zealand) is described in which Early Cretaceous high-pressure metamorphic rocks and virtually unmetamorphosed plutonic rocks occur. The dominant rocks are orthogneisses developed from synmetamorphic basic-intermediate intrusive complexes, the youngest and most widespread of which is the Early Cretaceous Western Fiordland Orthogneiss (WFO). The latter has undergone granulite facies metamorphism and occurs throughout much of western Fiordland. In the study area, the WFO protolith intruded a country rock of amphibolite facies metasediments and orthogeneisses. Fragments of the country rock are rafted within WFO and are represented by George Sound Paragneiss and Rafted Granitoid Gneisses. External country rock is represented by Arthur River Complex and Jagged Gneiss (possibly related to the Anita Ultramafites); it may also include Indecision Creek Complex and Mount Anau Complex. The George Sound Paragneiss is correlated with the Central Fiordland Metasediments of Oliver & Coggon.WFO was emplaced synkinematically in a subduction-related magmatic arc. A collisional event during of immediately following magma emplacement resulted in crustal thickening equivalent to onloading of a 20 km thick section over rocks already buried at mid-crustal depths. This event was responsible for peak load pressures of C. 12-13 kbar recorded throughout WFO and in at least part of the country rock. Intrusive relations and a shared metamorphic and structural history indicate that WFO granulites are not tectonically exotic relative to associated amphibolite facies rocks in western Fiordland.The steeply dipping Surprise Creek Fault juxtaposes high-pressure metamorphic rocks of western and central Fiordland against virtually unmetamorphosed gabbroic rocks of the Early Cretaceous Darran Complex. This structure is a major tectonic break and separates fundamentally different crustal levels of a disrupted Early Cretaceous magmatic arc. Arc magmatism, collision, and subsequent great uplift are key events controlling Mesozoic Fiordland evolution.

Regionally extensive two-pyroxene granulites in Fiordland, southwest New Zealand, are products of metamorphism of a suite of anhydrous magmas which crystallized two pyroxenes. The granulite protolith (igneous charnockitic rock) synkinematically intruded metasediment and other orthogneiss in an Early Cretaceous subduction-related magmatic arc, and during cooling experienced deformation-induced recrystallization to form granoblastic gneiss. The granulites occur side by side with coeval rocks of amphibolite facies. Mineral zoning and textural relationships in both granulites and amphibolite facies rocks provide evidence of two distinct periods of crystallization: 1) an early high temperature, comparatively low pressure event accompanying magmatic intrusion (andalusite-sillimanite facies series recorded locally in the country rock), followed by 2) high pressure metamorphism under conditions of ∼650°–700° C at ∼12–13 kbar. Garnet granulite locally overprinted earlier formed two-pyroxene granulite during the latter event. The pressure increase (∼6 kbar) between the two events is attributed to crustal thickening by overthrusting, and is equivalent to unloading of a ∼20 km thick slab over rocks already buried at mid-crustal depths. Both events occurred over a < 20 m.y. interval, between the time of magmatic emplacement of the granulite protolith and uplift-controlled final cooling of the terrain. The Phanerozoic granulites in Fiordland share some petrologic similarities with Precambrian granulite terrains, suggesting that at least some aspects of the former may serve as a useful model for development of the latter.

Regionally extensive two-pyroxene granulite facies orthogneisses of Early Cretaceous age in Fiordland, southwest New Zealand, are criss-crossed by garnet-bearing feldspathic veins (and dikes) having associated marginal reaction zones of garnet granulite. The two-pyroxene granulites resulted from fluid-absent meta-morphism of a suite of synkinematic primary anhydrous intrusions. Subsequent restricted formation of garnet granulite in feldspathic compositions, and locally eclogite in ultramafic compositions, proceeded chiefly via reactions involving hornblende breakdown, and occurred in response to sharply increased load pressure and local lowering of water activity. The restricted occurrence of carbonate scapolite and $$CO_{2}$$-rich fluid inclusions in the vein areas suggests that water activity was lowered by infiltrating carbonic fluids. Infiltration occurred along pre-existing fracture systems, many of which were already filled with plagio-clase-rich veins and dikes. Published carbon isotope data for $$CO_{2}$$ in scapolite within the veins indicates a mantle source. Open system behavior accompanying infiltration favored garnet stability by lowering bulk rock ferric/ferrous ratio and $$Na_{2}O$$ content. Fluid infiltration occurred at or near peak metamorphic pressure (~12 kbar at 650-700°C). Granulite metamorphism was of short duration (< 20 m.y.) and accompanied tectonic thickening in a subduction-related magmatic arc.

Sm-Nd and Rb-Sr isotopic analyses are reported for granulite facies orthogneisses from Fiordland southwest New Zealand. Whole-rock samples define a Rb-Sr isochron age of 120±15 Ma and an initial 87Sr/86Sr ratio of 0.70391±4. ɛNd values (at 120 Ma) show a relatively wide range of from −0.4 to 2.7 indicating decoupling of Sr-Nd isotope systems. Associated ultramafic rocks have initial 87Sr/86Sr ratios of from 0.70380 to 0.70430 and ɛNd values of from 0.1 to 3.0. The different initial ratios suggest that the various intrusions, although contemporaneous, were not derived through fractionation of a single parent magma. A metasedimentary enclave incorporated during emplacement of the granulitic rocks preserves a Proterozoic isotopic signature with a measured ɛNd(0) value of −10.2, 87Sr/86Sr ratio of 0.73679 and a TNd provenance age of 1490 Ma. The Rb-Sr whole rock age of the granulites is the same as obtained from recent U-Pb zircon dating (Mattinson et al. 1986) and is interpreted as the time of magmatic emplacement and essentially contemporaneous granulite facies metamorphism. Rb-Sr and Sm-Nd analyses of mineral systems indicate that the terrain had cooled below ∼300° C by ∼100 Ma providing further evidence that high grade metamorphism was of exceptionally short duration.

U-Pb isotopic analyses of zircons from a distinctive suite of previously undated granulite facies metaplutonic rocks, here termed the Western Fiordland Orthogneiss (WFO), in Fiordland, southwest New Zealand, indicate synkinematic magmatic emplacement between ∼120 and 130 Ma ago. These rocks were previously interpreted as possibly being of Precambrian age. Initial Pb and Sr ratios are consistent with arc/subduction related magmagenesis with little or no involvement of ancient continental crust. Subsequent high pressure (>12 kb) metamorphism of the WFO may reflect a major collision event involving crustal thickening by overthrusting of a >15 km thick sequence. Metamorphism ceased ≤116 Ma ago based on206Pb/238U ages of zircon from a retrogressed granulite. U-Pb isotopic analysis of apatite, along with previously published Rb/Sr mineral ages, indicate that final uplift and cooling to <300–400° C was largely completed by ∼90 Ma. The average uplift rate during this period is inferred to have been in excess of 1 mm/yr.

The phase relations of pyroxenes, amphiboles and associated minerals in metamorphic rocks of the Franciscan Complex can be graphically depicted on a ternary diagram which has at its apices the metamorphic clinopyroxene end members, viz NaAl-NaFe3+-Ca(Fe2+, Mg). Phases are plotted by projection from a constant subassemblage of minerals. This analysis allows interpretation of the effects of pressure, temperature, bulk rock composition and fluid composition on stability of minerals within the Franciscan.

A sequence of Jurassic rocks on Fidalgo Island, Washington, is interpreted to be ophiolite. The order of rock types, from the base upward, is serpentinite, layered gabbro, a dike complex made up mostly of plagiogranite, volcanic rocks that are dominantly keratophyre, coarse breccia with clasts of keratophyre and plagiogranite, pelagic argillite, and siltstone-sandstone turbidites. The plagiogranites and keratophyres have identical chemical compositions and are mutually gradational in field setting and textures, all of which suggests that they are cogenetic. These rocks are distinguished from calc-alkalic rock types by their very low content of K 2 O (where SiO 2 = 70%, K 2 O = 0.2% to 0.7%). Metasomatic alteration of the rocks appears to be insignificant, judging from (1) well-preserved primary igneous textures, (2) well-preserved primary intrusive and extrusive contacts, and (3) uniformity of chemical composition across igneous units. An oceanic origin of the ophiolite is suggested by the capping of pelagic sediments. Their fine grain size, abundance of radiolaria, and enrichment in Mn and other metals are virtually identical to those of modern Pacific pelagic sediments and unlike that of arc or epicontinental sediments. This interpretation conflicts with the apparent paucity of plagiogranite and keratophyre on the present-day sea floor. Field relations and chemical trends indicate that the plagiogranite-keratophyre magma is not the product of fractionation of the same melt that crystallized layered gabbro. High water content of the plagiogranite-keratophyre magma is indicated by hydrothermal alteration of the gabbro near plagiogranite intrusions and the occurrence of hornblende instead of pyroxene in mafic varieties. We suggest that this water is from the sea and that the anomalously low K 2 O content of these magmas is due to exchange with sea water.