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monly referred to as eclogites, but contrast As to the fate of the displaced SCB exposes mainly 1.6–1.8 Ga Yavapai-
with classic eclogites by having more Ca- LC-SCML, we suggest that several latest Mazatzal basement overlain by Proterozoic
and Mg-rich clinopyroxene, more Fe- and Oligocene lower lithosphere xenolith loca- to Mesozoic strata, much older ages are
Ca-rich garnet, and commonly contain tions from the Colorado Plateau transition expected. It should be noted that ca. 70–55
accessory hornblende. These arc root zone contain remnants of the missing litho- Ma porphyry copper deposits and a ca.
cumulates are commonly referred to as sphere. This assertion predicts that native 190–160 Ma magmatic arc crop out a few
“arclogites” (Anderson, 2005). sub-arc materials and displaced equivalents tens of kilometers SW of the studied xeno-
The preservation of >100 km of central should contain similar arrays of rock types. lith localities (GSA Data Repository Fig.
SNB lower lithosphere (arc root lower This is indeed the case, as ca. 25 Ma latite at DR1 ; e.g., Vikre et al., 2014; Tosdal and
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crust and upper mantle) contrasts sharply Camp Creek and Chino Valley localities Wooden, 2015; Chapman et al., 2018).
with the virtual absence of these materials each contain abundant nodules of, in order Hence, Latest Cretaceous–early Cenozoic
beneath the SCB (Fig. 1). Geochemical of decreasing abundance, garnet-pyroxene and Early–Middle Jurassic xenolith zircon
proxies for crustal thickness (e.g., Sr/Y rocks, garnet granulite, peridotite, and ages, readily distinguishable from Late
and La/Yb) strongly suggest that a deep quartzofeldspathic gneiss (Schulze and Cretaceous and Late Jurassic ages expected
sub-SCB root indeed existed prior to shal- Helmstaedt, 1979; Arculus and Smith, 1979; from the Mojave Desert, may point to a SW
low-angle subduction, forming in the Late Smith et al., 1994; Esperança et al., 1988, Arizona origin.
Jurassic and thickening significantly dur- 1997; Erdman et al., 2016). As with the sub-
ing the Late Cretaceous magmatic flare- SNB suite, garnet clinopyroxenite xenoliths NEW RESULTS AND
up (Howard et al., 2016). We now focus on from Chino Valley and Camp Creek are INTERPRETATIONS
the fate of missing SCB lower lithosphere arclogitic in composition, and equilibrated A small percentage (~5%) of garnet-clino-
and the regional extent of Farallon plate between 600 and 900 °C and 12–28 kbar pyroxenite-amphibole xenoliths contains
mantle lithosphere underplating beneath (45–100 km depth assuming a 2800 kg/m trace amounts of zircon. Zircon separated
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the “unrooted” SCB. overburden density; Smith et al., 1994; from host xenoliths were analyzed via laser
Esperança et al., 1988; Erdman et al., 2016). ablation–multi-collector inductively coupled
SCB LOWER LITHOSPHERE Furthermore, high field strength element plasma–mass-spectrometry (LA-MC-ICP-
DISPLACEMENT AND compositions and major element systemat- MS) at the Arizona LaserChron Center (see
RECONSTRUCTION ics of Arizona xenoliths balance those footnote 1).
It is important to reiterate here that expected from calc-alkaline magmatic dif- Three xenolith groups were identified,
despite LC-SCML removal from beneath ferentiation (Tang et al., 2018, 2019). Finally, based on lithologic relations and U-Pb zir-
the SCB, and subsequent tectonic under- Mesozoic plutons of the Mojave Desert and con ages. Group 1 xenoliths contain a sig-
plating of schist, the lithosphere-astheno- arclogite recovered from Chino Valley and nificant modal proportion of amphibole
sphere boundary in the Mojave region cur- Camp Creek all share similar isotope sys- (>10%), which overprints the primary
rently lies deeper than ~60 km (e.g., Luffi tematics, with Sr/ Sr and eNd values rang- arclogite assemblage, and are extensively
86
87
et al., 2009). This profound relationship ing from 0.706 to 0.711 and −2 to −10, injected and altered by host latite. Zircon
indicates that latest Cretaceous–Cenozoic respectively (Esperança et al., 1988; Smith grains extracted from these nodules yield a
reconstruction of the mantle lithosphere et al., 1994; Miller et al., 1996). These rela- bimodal age distribution consisting chiefly
beneath the schists must have taken place. tions point to a thick sub-SCB residue, of Late Jurassic (kernel density estimate
Relationships resolved in Dish Hill and rather than a lower plate, origin. peak at ca. 150 Ma) ages with a lower pro-
Crystal Knob xenolith suites (Fig. 1) indi- This assertion also predicts that materi- portion (~25%) of Late Cretaceous–early
cate that the underlying mantle lithosphere als once attached to the base of the SCB Cenozoic (peak at ca. 70 Ma) grains (Fig.
was reconstructed by tectonic underplating should share the early thermal history of 2). The second group of xenoliths consists
of Farallon plate sub-oceanic mantle the Mojave Desert. For example, the of relatively fresh arclogite (i.e., less
between 80 and 30 Ma (Luffi et al., 2009; Mojave Desert is underlain chiefly by injected with melt and containing less sec-
Liu et al., 2010; Quinn et al., 2018). Middle Jurassic–Early Cretaceous (ca. ondary amphibole). These samples contain
Considering that eclogitic fragments of the 160–140 Ma) and Late Cretaceous (90–70 zircon that yield a unimodal spread of con-
Farallon Plate (Usui et al., 2003) plus sig- Ma) arc plutonic assemblages with rela- cordant Cretaceous to early Cenozoic ages
nificant amounts of ancient LC-SCML tively small amounts of Mesozoic to ranging from ca. 100–50 Ma with a peak
both underlie the Laramide interior, the Neoproterozoic metasedimentary rocks centered at ca. 75 Ma (Fig. 2). A final group
lower lithosphere beneath a significant part (Wells and Hoisch, 2008; Barth et al., 2008; of mid- to deep-crustal foliated granitic
of the Laramide corridor must be a com- Needy et al., 2009; Chapman et al., 2018). gneiss is less abundant than its deep-crust/
posite of these assemblages. As shown on Hence, if xenoliths recovered from Chino upper mantle arclogite counterparts and
Figure 1, the underplated schists plus Valley and Camp Creek localities are yields concordia ages of ca. 1.7 Ga (Fig.
underlying mantle lithosphere constitute a indeed consanguineous with the SCB, the DR7 [see footnote 1]). These nodules are
lithosphere-scale accretionary complex xenoliths should yield chiefly Late interpreted as Proterozoic assemblages
lying beneath a carapace of SCB granitoids Cretaceous and Late Jurassic ages. If native to central Arizona.
that was stripped of most of its underlying instead the xenoliths are native to the Our new results indicate that studied
mantle wedge. Colorado Plateau transition zone, which arclogitic xenoliths are coeval with the

1 GSA data repository item 2019368, field observations, sample descriptions, analytical methods, and zircon U-Pb data, is online at www.geosociety.org/datarepository/2019.

6 GSA Today | January 2020