Northern Cascade Arc Research Articles

Temporal and spatial evolution of Northern Cascade Arc magmatism revealed by LA–ICP–MS U–Pb zircon dating

We present thirty new laser ablation inductively coupled plasma mass spectrometry U–Pb zircon dates for intermediate to silicic plutons of the Northern Cascade Arc with emphasis on the Chilliwack batholith – Mount Baker magmatic focus, located in southwestern British Columbia and northern Washington. Chilliwack magmatism commenced at ∼35 Ma in southwestern British Columbia and the most voluminous plutons define a cluster at ∼32–29 Ma, documenting an early flare-up. During the same interval, the Index, Squire Creek, and Cascade Pass intrusions were emplaced south of the Chilliwack batholith. North of the Chilliwack, maximum pluton ages become progressively younger northward, tracking the northerly migration of the edge of the Farallon–Juan de Fuca–Explorer plate system relative to North America. Chilliwack magmatism continued from ∼29 Ma to 22 Ma at a slightly reduced flux, followed by a lull from 22 to 11 Ma during which magmatism shifted north to the Mount Barr batholith (18 Ma). Chilliwack magmatism resumed by 11 Ma but was intermittent and the intrusive flux was significantly lower. The temporal decrease in intrusive flux displayed by the Chilliwack batholith correlates with the declining convergence rate of the Juan de Fuca plate since arc inception. The 11 Ma-to-present magmatism extends a pattern of southwesterly migration of the magmatic focus previously identified from ∼4 Ma (Hannegan caldera) to the modern Mount Baker volcanic field. Crustal rotation accounts for the rate of the first ∼7 million years of migration. However, the migration rate more than doubled at ∼4 Ma, coinciding with separation of the Explorer plate and initiation of Juan de Fuca plate rollback.

Magnesium Isotopes as a Tracer of Crustal Materials in Volcanic Arc Magmas in the Northern Cascade Arc

Fifteen North Cascade Arc basalts and andesites were analyzed for Mg isotopes to investigate the extent and manner of crustal contributions to this magmatic system. The δ26Mg of these samples vary from within the range of ocean island basalts (the lightest being -0.33 ± 0.07‰) to heavier compositions (as heavy as -0.15 ± 0.06‰). The observed range in chemical and isotopic composition is similar to that of other volcanic arcs that have been assessed to date in the circum-pacific subduction zones and in the Caribbean. The heavy Mg isotope compositions are best explained by assimilation and fractional crystallization within the deep continental crust with a possible minor contribution from the addition of subducting slab-derived fluids to the primitive magma. The bulk mixing of sediment into the primitive magma or mantle source and the partial melting of garnet-rich peridotite are unlikely to have produced the observed range of Mg isotope compositions. The results show that Mg isotopes may be a useful tracer of crustal input into a magma, supplementing traditional methods such as radiogenic isotopic and trace element data, particularly in cases in which a high fraction of crustal material has been added.

Using mineral geochemistry to decipher slab, mantle, and crustal input in the generation of high-Mg andesites and basaltic andesites from the northern Cascade Arc

To better understand the role of slab melt in the petrogenesis of North Cascades magmas, this study focuses on petrogenesis of high-Mg lavas from the two northernmost active volcanoes in Washington. High-Mg andesites (HMA) and basaltic andesites (HMBA) in the Cascade Arc have high Mg# [molar Mg/(Mg+Fe 2+ )] relative to their SiO 2 contents, elevated Nd/Yb, and are Ni- and Cr-enriched. The rock units examined here include the Tarn Plateau HMBA (51.8–54.0 wt% SiO 2 , Mg# 68–70) and Glacier Creek HMA (58.3–58.7 wt% SiO 2 , Mg# 63–64) from the Mount Baker Volcanic Field, and the Lightning Creek HMBA (54.8–54.6 SiO 2 , Mg# 69–73) from Glacier Peak. This study combines major and trace element compositions of minerals and whole rocks to test several petrogenetic hypotheses and to determine which, if any, are applicable to North Cascades HMA and HMBA. In the Tarn Plateau HMBA, rare earth element (REE) equilibrium liquids calculated from clinopyroxene compositions have high Nd/Yb that positively correlates with Mg#. This correlation suggests an origin similar to that proposed for Aleutian adakites, where intermediate, high Nd/Yb slab-derived melts interact with the overlying mantle to become Mg-rich, and subsequently mix with low Nd/Yb, mantle-derived mafic magmas with lower Mg#. In the Glacier Creek HMA, elevated whole-rock MgO and SiO 2 contents resulted from accumulation of xenocrystic olivine and differentiation processes, respectively, but the cause of high Nd/Yb is less clear. However, high whole-rock Sr/P (fluid mobile/fluid immobile) values indicate a mantle source that was fluxed by an enriched, hydrous slab component, likely producing the observed high Nd/Yb REE signature. The Lightning Creek HMBA is a hybridized rock unit with at least three identifiable magmatic components, but only one of which has HMA characteristics. Cr and Mg contents in Cr-spinel and olivine pairs in this HMA component suggest that its source is a strongly depleted mantle, and high whole-rock Sr/P values indicate mantle melting that was induced through hydration, likely adding the component responsible for the observed high Nd/Yb REE pattern. The elevated SiO 2 contents (54.6 wt%) of the HMA component resulted from differentiation or high degrees of partial melting of ultramafic material through the addition of H 2 O. Therefore the Lightning Creek HMBA is interpreted to have originated from a refractory mantle source that underwent melting through interaction with an enriched slab component. Our results indicate that in addition to slab-derived fluids, slab-derived melts also have an important role in the production of HMA and HMBA in the north Cascade Arc.

Evidence for trench-parallel mantle flow in the northern Cascade Arc from basalt geochemistry

Geochemical data for basalts from the Garibaldi Volcanic Belt (northern segment of the Cascade Arc) define arc-parallel gradients in trace elements and isotope ratios that extend at least 150 km into the arc from the northern margin of the subducting Juan de Fuca plate. Southerly increases in Zr/Nb, Ba/Nb, Th/La, Pb/Ce, 208Pb/204Pb and 176Hf/177Hf indicate greater mantle depletion and higher slab-derived contributions with distance from the slab edge. Temperatures and pressures of mantle melt segregation also decrease to the south. The gradients are most plausibly explained as a consequence of slab rollback-induced toroidal flow at the northern slab edge (Nootka fault zone), whereby enriched (OIB-type) NE Pacific asthenospheric mantle from beneath the slab is drawn into the mantle wedge in a trench-parallel southerly flow pattern. Melts of the enriched asthenosphere are progressively diluted to the south by melts of the slab-modified, depleted mantle wedge. Arc-parallel changes in slab thermal conditions cannot account for these gradients. Trench parallel geochemical gradients in the northern Cascade Arc are consistent with shear wave splitting data, numerical modeling, and experimental studies showing that trench-parallel mantle flow may be a common phenomenon near slab edges and slab gaps.

Origin of Basalts in a Hot Subduction Setting: Petrological and Geochemical Insights from Mt. Baker, Northern Cascade Arc

The northern Cascade arc is an end-member ‘hot’ subduction zone where slab dehydration may be essentially complete prior to reaching sub-arc depths, presenting a potential problem for a flux melting origin for arc basalts. Nevertheless, mafic lavas from the Mt. Baker volcanic field, the most magmatically productive volcano in the northern Cascade arc, record subduction input and compositions typical of calc-alkaline magmas. Relative to normal mid-ocean ridge basalt, the most primitive lavas at Mt. Baker show elevated abundances of large ion lithophile elements (LILE), Pb and light rare earth elements (LREE). High field strength elements (HFSE) and heavy REE (HREE) are depleted relative to LILE. Reconstructed primary magmas define three groups: Group I calcalkaline basalts (Sulphur Creek, Lake Shannon, Hogback) have lower SiO2, (La/Yb)N and LILE/HFSE, and are olivineand plagioclase-phyric; Group II high-Mg basaltic andesites (Tarn Plateau, Cathedral Crag) also contain clinopyroxene phenocrysts and have higher H2O, SiO2, (La/Yb)N and LILE/HFSE; Group III low-K olivine tholeiite (Park Butte) has the lowest (La/Yb)N but highest LILE/HFSE.The redox states of all groups lie between QFM (quartz^fayalite^magnetite) and QFM þ1·4. Pb, Sr and Nd isotope compositions are similar among all the analysed samples and are consistent with a depleted mantle source modified by input from the subducting slab. Trace element^isotope modeling indicates a subduction component composed predominantly of metabasaltderived fluid with lesser amounts of sediment melt and metabasalt melt. Group I basalts record the smallest melt fractions ( 5^7%), lowest water contents (1·5^2·1wt %), and highest temperatures and pressures of mantle segregation (up to 13548C, 1·5 GPa) from lherzolitic ( spinel) residues. Group II basaltic andesites show the greatest extents of mantle metasomatism, the highest water contents (2·7^3·7 wt %) and partial melt fractions (10^12%), and segregated from harzburgite at 12708C, 1GPa, consistent with pooling of melts at the Moho. Group III records P^Tconditions similar to Group I ( 1·4 GPa, 13268C) but melt fractions (12%) and mantle residues (harzburgite) are more similar to Group II, and H2O contents ( 2·1wt %) are intermediate. Melting beneath Mt. Baker was initiated by dehydration melting of amphibole peridotite at 95 km and 10208C, within the stability field of garnet lherzolite. Initial melt fractions were small ( 1^2%) and near water-saturation. Phase equilibria and trace element modeling show no evidence for garnet-bearing mantle residues, indicating that progressive melting during ascent of diapirs through the hot core of the convecting mantle wedge reduced H2O contents and erased any residual garnet signature. Because fluid release from the slab is restricted to the forearc, mantle hydrated at shallow depths in the serpentine and/or chlorite stability fields must be down-dragged to the region of amphibole stability to initiate dehydration melting.

Sr‐Nd‐Hf‐Pb isotope and trace element evidence for the origin of alkalic basalts in the Garibaldi Belt, northern Cascade arc

In the Garibaldi Belt, the northern segment of the Cascade arc, basalts at Bridge River Cones, Salal Glacier, and Mt. Meager (BSM volcanic centers) are alkalic, atypical for an arc setting. Subduction signatures are negligible or absent from primitive alkalic basalts from Salal Glacier and Bridge River, while altered oceanic crust may have contributed a minimal amount of fluid at Mt. Meager. More evolved BSM basalts display trace element signatures considered typical of arc lavas, but this is a consequence of deep crustal assimilation rather than primary input from the subducted slab. Primary BSM basalts represent 3–8% melts that segregated from enriched garnet lherzolite at significantly higher temperatures and pressures (70–105 km) than calc‐alkaline Cascade arc basalts. The BSM mantle source is significantly more incompatible element‐enriched than the depleted mantle tapped by calc‐alkaline Cascade arc basalts. The BSM basalts are also isotopically distinct from calc‐alkaline Cascade arc basalts, more similar to MORB and intraplate basalts of the NE Pacific and NW North America. The relatively deep, hot, and geochemically distinct mantle source for BSM basalts is consistent with upwelling asthenosphere. The BSM volcanic centers are close to the projected trace of the Nootka fault, which forms the boundary between the subducting Juan de Fuca plate and the near‐stagnant Explorer plate. A gap or attenuated zone between the plates may promote upwelling of enriched asthenosphere that undergoes low‐degree decompression melting to generate alkalic basalts that are essentially free of slab input yet occur in an arc setting.

Mafic magmas from Mount Baker in the northern Cascade arc, Washington: probes into mantle and crustal processes

Five mafic lava flows located on the southern flank of Mount Baker are among the most primitive in the volcanic field. A comprehensive dataset of whole rock and mineral chemistry reveals the diversity of these mafic lavas that come from distinct sources and have been variably affected by ascent through the crust. Disequilibrium textures present in all of the lavas indicate that crustal processes have affected the magmas. Despite this evidence, mantle source characteristics have been retained and three primitive endmember lava types are represented. These include (1) modified low-K tholeiitic basalt (LKOT-like), (2) typical calc-alkaline (CA) lavas, and (3) high-Mg basaltic andesite and andesite (HMBA and HMA). The Type 1 endmember, the basalt of Park Butte (49.3–50.3 wt% SiO2, Mg# 64–65), has major element chemistry similar to LKOT found elsewhere in the Cascades. Park Butte also has the lowest overall abundances of trace elements (with the exception of the HREE), indicating it is either derived from the most depleted mantle source or has undergone the largest degree of partial melting. The Type 2 endmember is represented by the basalts of Lake Shannon (50.7–52.6 wt% SiO2, Mg# 58–62) and Sulphur Creek (51.2–54.6 wt% SiO2, Mg# 56–57). These two lavas are comparable to calc-alkaline rocks found in arcs worldwide and have similar trace element patterns; however, they differ from each other in abundances of REE, indicating variation in degree of partial melting or fractionation. The Type 3 endmember is represented by the HMBA of Tarn Plateau (51.8–54.0 wt% SiO2, Mg# 68–70) and the HMA of Glacier Creek (58.3–58.7 wt% SiO2, Mg# 63–64). The strongly depleted HREE nature of these Type 3 units and their decreasing Mg# with increasing SiO2 suggests fractionation from a high-Mg basaltic parent derived from a source with residual garnet. Another basaltic andesite unit, Cathedral Crag (52.2–52.6 wt% SiO2, Mg# 55–58), is an Mg-poor differentiate of the Type 3 endmember. The calc-alkaline lavas are least enriched in a subduction component (lowest H2O, Sr/PN, and Ba/Nb), the LKOT-like lavas are intermediate (moderate Sr/PN and Ba/Nb), and the HMBA are most enriched (highest H2O, Sr/PN and Ba/Nb). The generation of the LKOT-like and calc-alkaline lavas can be successfully modeled by partial melting of a spinel lherzolite with variability in composition of slab flux and/or mantle source depletion. The HMBA lavas can be successfully modeled by partial melting of a garnet lherzolite with slab flux compositionally similar to the other lava types, or less likely by partial melting of a spinel lherzolite with a distinctly different, HREE-depleted slab flux.

Mantle flow through the Northern Cordilleran slab window revealed by volcanic geochemistry

The Northern Cordilleran slab window formed beneath western Canada concurrently with the opening of the Californian slab window beneath the southwestern United States, beginning in Late Oligocene–Miocene time. A database of 3530 analyses from Miocene– Holocene volcanoes along a 3500-km-long transect, from the northern Cascade Arc to the Aleutian Arc, was used to investigate mantle conditions in the Northern Cordilleran slab window. Using geochemical ratios sensitive to tectonic affi nity, such as Nb/Zr, we show that typical volcanic arc compositions in the Cascade and Aleutian systems (derived from subduction-hydrated mantle) are separated by an extensive volcanic fi eld with intraplate compositions (derived from relatively anhydrous mantle). This chemically defi ned region of intraplate volcanism is spatially coincident with a geophysical model of the Northern Cordilleran slab window. We suggest that opening of the slab window triggered upwelling of anhydrous mantle and displacement of the hydrous mantle wedge, which had developed during extensive early Cenozoic arc and backarc volcanism in western Canada. High heat fl ow throughout the western Canadian Cordillera is broadly coincident with the fi eld of intraplate volcanism and is linked to slab window-induced mantle upwelling.

The generation of a diverse suite of Late Pleistocene and Holocene basalt through dacite lavas from the northern Cascade arc at Mount Baker, Washington

Mt. Baker is a dominantly andesitic stratovolcano in the northern Cascade arc. In this study, we show that the andesites are not all derived from similar sources, and that open-system processes were dominant during their petrogenesis. To this end, we discuss petrographic observations, mineral chemistry, and whole rock major and trace element chemistry for three of Mt. Baker’s late Pleistocene to Holocene lava flow units. These include the basalt and basaltic andesite of Sulphur Creek (SC) (51.4–55.8 wt% SiO2, Mg# 57–58), the Mg-rich andesite of Glacier Creek (GC) (58.3–58.7 wt% SiO2, Mg# 63–64), and the andesite and dacite of Boulder Glacier (BG) (60.2–64.2 wt% SiO2, Mg# 50–57). Phenocryst populations in all units display varying degrees of reaction and disequilibrium textures along with complicated zoning patterns indicative of open-system processes. All lavas are medium-K and calc-alkaline, but each unit displays distinctive trace element and REE characteristics that do not correlate with the average SiO2 content of the unit. The mafic lavas of SC have relatively elevated REE abundances with the lowest (La/Yb)N (~4.5). The intermediate GC andesites (Mg- and Ni-rich) have the lowest REE abundances and the highest (La/Yb)N (~6.7) with strongly depleted HREE. The more felsic BG lavas have intermediate REE abundances and (La/Yb)N (~6.4). The high-Mg character of the GC andesites can be explained by addition of 4% of a xenocrystic olivine component. However, their depleted REE patterns are similar to other high-Mg andesites reported from Mt. Baker and require a distinct mantle source. The two dominantly andesitic units (GC and BG) are different enough from each other that they could not have been derived from the same parent basalt. Nor could either of them have been derived from the SC basalt by crystal fractionation processes. Crystal fractionation also cannot explain the compositional diversity within each unit. Compositional diversity within the SC unit (basalt to basaltic andesite) can, however, be successfully modeled by mixing of basalt with compositions similar to the dacites in the BG unit. Given that the BG dacites erupted at ~80–90 ka, and a similar composition was mixed with the SC lavas at 9.8 ka, the process that produced this felsic end-member must have been repeatedly active for at least 70 ka.