Deciphering magmatic processes in calc-alkaline plutons using

57 Pages·2015·1.74 MB·English

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1
2 Deciphering magmatic processes in calc-alkaline plutons using trace element
3 zoning in hornblende
4 Revision 1.
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6 Calvin G. Barnes1*, Valbone Memeti2, Nolwenn Coint3
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8 1Department of Geosciences, Texas Tech University, Lubbock, TX 07409-1053, U.S.A.
9 2Department of Geological Sciences, California State University, Fullerton, Fullerton, CA
10 92834
11 3Geological Survey of Norway, PO Box 6315 Sluppen, 7491 Trondheim, Norway
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13 *E-mail: [email protected]
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16 ABSTRACT
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18 Hornblende in the Kuna Crest lobe (KCL) of the Tuolumne Intrusive Complex (TIC) and
19 the upper zone of the Wooley Creek batholith (WCB) precipitated over a temperature range of
20 ~835 to 700°C, and thus has the potential to record magmatic processes. We measured trace
21 element concentrations in hornblende from the WCB, from the KCL of the TIC, and from one
22 sample from an adjacent interior unit of the TIC in order to compare and contrast magmatic
23 processes in these two mid-crustal intrusions. In both systems the magmatic amphibole is
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24 magnesiohornblende in which Ti, Zr, Hf, Nb, Sr, Ba and rare earth elements (REE) typically
25 decrease from crystal interiors to rims, an indication of compatible behavior of these elements,
26 and the size of the negative Eu anomaly decreases. In the Kuna Crest lobe, hornblende from
27 individual mapped units differs in trace element abundances and zoning trends. Some samples
28 contain at least two distinct hornblende populations, which is particularly evident in the shapes
29 of REE patterns. In contrast, compositions of hornblende from all structural levels of the upper
30 WCB and related dacitic roof-zone dikes form a single broad array and the REE patterns are
31 essentially indistinguishable, regardless of rock type, from quartz diorite to granite. In the WCB,
32 Zr/Hf ratios in hornblende are consistent with crystallization from a melt with chondritic Zr/Hf
33 values. In contrast, most hornblende in the KCL has Zr/Hf values lower than expected from
34 crystallization from a melt with chondritic values, suggesting that zircon fractionation occurred
35 before and during crystallization of the hornblende. Simple fractional crystallization models
36 indicate that REE, high field strength elements, Sr, and Ba were compatible in KCL and WCB
37 magmas as hornblende grew; these trends require removal of hornblende + plagioclase + zircon
38 ± ilmenite ± biotite.
39 The uniform variations of trace element concentrations and patterns in the upper WCB
40 and roof-zone dikes indicates crystallization from a large magma body that was compositionally
41 uniform; probably stirred by convection caused by influx of mafic magmas at the base of the
42 zone (Coint et al., 2013a, b; cf. Burgisser and Bergantz, 2011). In contrast, in the KCL, each
43 analyzed sample displays distinct hornblende compositions and zoning patterns, some of which
44 are bimodal. These features indicate that each analyzed sample represents a distinct magma and
45 that individual magmas were variably modified by fractionation and mixing. Hornblende trace
46 element contents and zoning patterns prove to be powerful tools for identification of magma
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47 batches and for assessing magmatic processes, and thereby relating plutonic rocks to hypabyssal
48 and volcanic equivalents.
49 INTRODUCTION
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51 Results from U–Pb zircon geochronology (e.g., Mattinson, 2005; Coleman et al., 2004;
52 Matzel et al., 2006; Michel et al., 2008; Schaltegger et al., 2009; Schoene et al., 2010; 2012; and many
53 others) indicate that some batholith-scale magma systems are emplaced over time scales longer
54 than thermal models predict for the longevity of individual magma batches (e.g., Tappa et al.,
55 2011; Paterson et al., 2011). These results have led to the interpretation that large plutons are
56 incrementally emplaced, thus raising a number of questions: (1) are individual magma batches
57 related to one another? That is, did they form by some combination of fractional crystallization,
58 crustal melting, magma mixing, and assimilation such that one magma batch can be related to
59 another? (2) Regardless of petrogenesis, do magma batches interact at the level of emplacement,
60 allowing for blending of melts and entrained crystals (e.g., Matzel et al., 2006; Miller et al.,
61 2007; Memeti et al., 2010), or is each batch a separate intrusive entity (e.g., Coleman et al., 2004,
62 2012; Glazner et al., 2004)? (3) If magma batches mix, is it on a local or pluton-wide scale? The
63 presence of large volumes of interconnected melt are seemingly required to explain
64 homogeneous ignimbrites (Bachmann et al., 2005; Christiansen, 2005; Gelman et al., 2013),
65 large-scale low velocity anomalies in the middle to upper crust in modern volcanic arcs (Brasse
66 et al., 2002; Yuan et al., 2000; 2013; Ward et al., 2014), preservation of continuous hypersolidus
67 structures in large plutons (Paterson et al., 1998; Zak et al., 2007), and the lack of observable
68 sharp contacts that might form between individual incrementally emplaced magmas within
69 seemingly homogeneous intrusive units (Memeti et al., 2010, but see Bartley et al., 2008, 2012).
70 Thus, particularly in large plutons that appear to be homogeneous, it is desirable to identify a
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71 means by which individual magma batches might be identified in terms of physical, chemical, or
72 mineralogical features. Moreover, if distinct magma batches mix, identification of components
73 from the mixing end members is required to determine the spatial scales of mixing.
74 Any study of plutonic rocks must also be concerned with the relationship between bulk-
75 rock compositions and the composition of melts from which the rocks crystallized. It may be
76 possible to use bulk-rock compositions to identify groups of rocks that belong to a particular
77 magma batch. However, the effects of crystal accumulation, melt percolation, and convection
78 would obscure the identities of individual magma batches that existed. An alternative approach
79 to using bulk rock composition as a proxy for the melt is to infer melt compositions from rock-
80 forming minerals that reliably record the composition of the melt(s). Such records of melt
81 composition are not affected by crystal accumulation.
82 This contribution is a reconnaissance of trace element compositions of hornblende from
83 the Kuna Crest lobe (KCL) of the Tuolumne Intrusive Complex (TIC; Sierra Nevada batholith,
84 California) and from the upper zone of the Wooley Creek batholith (WCB; Klamath Mountains,
85 California). We chose these two systems because they are well mapped, have similar ranges of
86 bulk composition, and have similar mineral assemblages. Our goal is to determine whether
87 hornblende from individual intrusive units that make up large batholiths can be distinguished
88 from one another and to learn whether intracrystalline zoning patterns can be used to interpret
89 magmatic processes. We focus on hornblende, because it is commonly a near-liquidus phase in at
90 least some of the magmas of interest (e.g., Piwinskii, 1973a, b; Naney, 1983, Johnson and
91 Rutherford, 1989; Schmidt, 1992) and it incorporates many trace elements in concentrations high
92 enough to be precisely analyzed. Mineral/melt partition coefficients have been determined for
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93 many of the trace elements of interest (e.g. Sisson, 1994; Klein et al, 1997; Brophy et al., 2011;
94 see review in Tiepolo et al., 2007).
95 We find that hornblende from individual units of the KCL is distinct and that
96 intracrystalline zoning patterns provide information about petrogenetic processes that affected
97 individual batches. In contrast, trace element contents and zoning in hornblende from numerous
98 samples of the upper WCB cannot be distinguished from one another, suggesting either early
99 homogenization of the large upper zone magma (≥ 160 km3) or virtually identical petrologic
100 history in numerous magma batches prior to emplacement.
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102 GEOLOGIC SETTING
103
104 Tuolumne Intrusive Complex
105 The Kuna Crest lobe is part of the 1,100 km2, 95–85 Ma Tuolumne Intrusive Complex
106 (TIC), which is exposed in the eastern Sierra Nevada batholith, central California (Fig. 1;
107 Bateman and Chappell, 1979; Kistler and Fleck, 1994, see also Memeti et al., 2010, 2014, this
108 volume). The TIC is one of four large composite intrusive complexes or suites emplaced near the
109 end of the Sierran Cretaceous magma flare-up (Kistler et al., 1986; Coleman and Glazner, 1997;
110 Ducea, 2001). The TIC is an intermediate to felsic composition, calc-alkaline, magnetite series
111 intrusive complex that, based on Al-in-hornblende barometry, was emplaced at 8–10 km
112 paleodepth and was not tilted after emplacement (Ague and Brimhall, 1988; Memeti et al.,
113 2009). The complex is zoned inward and toward the northeast (Figs. 1, 2) from the oldest and
114 most mafic units to younger and more felsic ones (Memeti et al., 2010). The most leucocratic
115 rocks consist of the fine-grained leucocratic Johnson Granite Porphyry and small lenses within
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116 the Cathedral Peak Granodiorite (Bateman and Chappell, 1979; Kistler and Fleck, 1994; Memeti
117 et al., 2010, 2014). Internal contacts between these units vary between sharp and gradational and
118 commonly show zones of hybridization (Fig. 1; Žák and Paterson, 2005).
119 The oldest and most mafic units of the TIC are the quartz diorite of May Lake, the
120 tonalites of Glacier Point and Grayling Lake, and the granodiorite of Kuna Crest (Bateman and
121 Chappell, 1979; Bateman, 1992). The most extensive exposures are in a compositionally-zoned,
122 approximately 8 by 10 km (80 km2), body that forms the southeastern extension of the TIC that
123 was mapped as the granodiorite of Kuna Crest (Bateman and Chappell, 1979). This unit is here
124 referred to as the Kuna Crest lobe (KCL; Figs. 1, 2). The KCL is mostly equigranular, fine- to
125 medium-grained granodiorite, tonalite, and diorite, although gabbroic rocks crop out in a sheeted
126 complex along the lobe margin and elsewhere (Fig. 2; Memeti et al., this volume). Zones I, II
127 and III are the main intrusive units of the KCL; they are separated by gradational contacts (Fig.
128 2; Memeti et al., 2010, this volume) and vary mainly in texture, color, and grain size. The
129 southeastern satellite plutons around Waugh Lake and Thousand Island Lake (Fig. 2) are
130 granodioritic to leucogranitic (Memeti et al., this volume). All Kuna Crest rocks contain anhedral
131 to subhedral biotite and hornblende, are rich in plagioclase, and have variable amounts of alkali-
132 feldspar (orthoclase or microcline) and quartz. The rocks contain accessory Fe-Ti oxides and
133 zircon, and most contain titanite and allanite. Internal contacts are sharp to gradational.
134 Magmatic fabrics tend to be strong and are locally associated with subsolidus fabrics and cm-
135 scale shear zones (Žák et al., 2007).
136 Transition from the Kuna Crest lobe to the equigranular Half Dome Granodiorite (eHD)
137 is via a hybrid zone that contains crystals typical of each unit (tHD in Fig. 2). Subhedral
138 hornblende typical of the Kuna Crest Granodiorite occurs with larger, euhedral hornblende (≤2
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139 cm long), euhedral biotite books (≤1 cm in diameter) and titanite (≤1 cm long) typical of the Half
140 Dome Granodiorite (Memeti et al., 2010; this volume). The eHD in turn has gradational to sharp
141 contacts inward to the porphyritic Half Dome Granodiorite. This unit contains hornblende and
142 titanite similar to the eHD, but has lower color index, and contains 1–3 cm-long alkali feldspar
143 phenocrysts. The next major unit inward is the Cathedral Peak Granodiorite (CP), which also
144 encompasses granite and leucogranite. The modal abundances of biotite and titanite in the CP are
145 significantly lower, hornblende is sparse, and quartz and alkali feldspar abundances are higher
146 compared to the porphyritic Half Dome Granodiorite. Alkali feldspar megacrysts locally range to
147 12–15 cm in length. The Kuna Crest and Half Dome units contain abundant centimeter- to
148 decimeter-scale mafic, microgranular, locally plagioclase-phyric enclaves, which decrease in
149 abundance and size in pHD and are scarce to absent in the CP.
150 High precision CA-ID-TIMS (chemical abrasion-isotope dilution- thermal ionization
151 mass spectrometry) U-Pb (zircon) ages reveal that crystallization of the KCL occurred at
152 94.9±0.3 Ma along the margin and ceased at 92.9±0.1 Ma in the Kuna Crest–Half Dome hybrid
153 zone (Memeti et al., 2010; this volume); these ages essentially span the entire crystallization
154 history of the Kuna Crest Granodiorite throughout the TIC. The equigranular Half Dome
155 Granodiorite in the main TIC crystallized between 92–90 Ma, the porphyritic Half Dome
156 Granodiorite at 90–88 Ma, and the Cathedral Peak Granodiorite at 88–85 Ma (age data
157 summarized in Memeti et al., 2010).
158 Wooley Creek batholith
159 The Wooley Creek batholith (WCB) is a Late Jurassic pluton in the Klamath Mountains
160 geologic province, northern California (Fig. 3). Post-emplacement tilting to the southwest
161 followed by exhumation exposed at least 9 km of structural relief through the pluton (Barnes et
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162 al., 1986b). The pluton consists of a northeastern lower zone of biotite hornblende two-pyroxene
163 gabbro through tonalite and a southwestern upper zone of biotite hornblende tonalite to granite
164 (Fig. 3; Barnes, 1983; Coint et al., 2013a). Dacitic to rhyodacitic dikes (roof-zone dikes) intrude
165 the structurally highest parts of the aureole and are interpreted to have leaked from underlying
166 upper zone magmas. The upper zone underlies ~160 km2 and represents a single magma body
167 that underwent internal differentiation (Barnes et al., 1986a; Coint et al., 2013a, b). Granite and
168 granodiorite of the upper zone are separated from the host rocks by mafic selvages in the NW
169 and south (Fig. 3); the former is coeval and compositionally associated with the lower zone,
170 whereas the latter is coeval with the upper zone (Coint et al., 2013a). The lower and upper zones
171 are locally separated by the central zone, which was the site of emplacement of sheet-like bodies
172 of upper and lower zone magmas, injection of synplutonic mafic dikes, and intense magma
173 mixing and mingling (Coint et al., 2013a).
174 Emplacement ages (CA-ID-TIMS U-Pb, zircon; Coint et al., 2013a) for the lower zone
175 are 158.99±0.17 and 159.22±0.10 Ma, for the upper zone are 158.21±0.17 and 158.25±0.46 Ma,
176 and for the central zone are 158.30±0.16 and 159.01±0.20 Ma (all uncertainties are 2σ). Thus,
177 although the upper and lower zones are temporally distinct, the central zone contains rocks
178 whose ages indicate the presence of both upper and lower zone magmas. More significantly, the
179 presence of sheets of upper-zone-like rocks in the central zone indicates growth of the upper
180 zone reservoir by emplacement of multiple magma batches. A late-stage biotite hornblende
181 granite, unrelated to the upper zone, was emplaced in the southern part of the batholith at
182 155.6±1.19 Ma (op. cit.).
183 Although lower-zone rocks contain magmatic hornblende, most such hornblende grew in
184 reaction relationship with pyroxene, and in many cases inherited its trace element signature from
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185 the pyroxene (Coint et al., 2013b). Therefore, this study is focused on hornblende from the upper
186 zone because it crystallized directly from a melt phase and because fractionation of hornblende
187 occurred in the upper WCB magma (Barnes, 1983, 1987). The upper WCB is gradationally
188 zoned from quartz diorite and tonalite in its eastern (lowest) part to granodiorite, quartz
189 monzonite, and granite in its western (highest) part (Fig. 3). On the basis of Al-in-hornblende
190 barometry (Schmidt, 1992), the transition from the central zone to the upper part of the upper
191 zone represents 3500–5000 m of structural relief (9–14.5 km paleodepths). Mafic microgranular
192 enclaves commonly occur in swarms in the structurally lowest part of the upper zone and, while
193 ubiquitous, decrease in abundance upward (westward; Barnes, 1983; Coint et al., 2013a).
194 Zonation of the upper zone was explained as the result of crystal-liquid separation via upward
195 percolation of melt-rich magma (Coint et al, 2013a, b).
196
197 PETROGRAPHY
198
199 Kuna Crest lobe and adjacent eHD
200 The analyzed Kuna Crest lobe samples (petrographic descriptions in Supplemental
201 Appendix 1) range from fine to coarse grained and are hypidiomorphic-granular tonalite and
202 granodiorite (Fig. 4). Brown biotite and olive to green hornblende are the principal mafic
203 minerals. Some samples contain augite rimmed by hornblende; the augite cores are variably
204 altered to pale green amphibole ± quartz ± Fe-Ti oxides. Thus, hornblende occurs as individual
205 prismatic grains and as rims on augite (Fig. 4A). Some hornblende is replaced by blue-green
206 actinolitic amphibole (Fig. 4E). Sparse glomerocrysts consist of biotite ± hornblende ± relict
207 augite ± Fe-Ti-oxides ± titanite ± plagioclase. Plagioclase is variably euhedral to anhedral and
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208 typically shows oscillatory-normal zoning (~An ); some grains have rims as sodic as An .
46–33 20
209 Quartz and alkali-feldspar are interstitial. If titanite is present in KCL samples, it is generally
210 interstitial, but the sample from the hybrid unit in the interior (KCL-536, Fig. 2) contains both
211 interstitial and prismatic (up to 0.5 cm long) titanite crystals. Other accessory minerals are
212 apatite, magnetite ± ilmenite, zircon and rare allanite; epidote and chlorite are secondary.
213 Centimeter- to meter-scale porphyritic mafic enclaves with fine-grained groundmass are
214 exposed in the hybrid granodiorite of the KCL (tHD, Fig. 2). The analyzed mafic enclave (KCL-
215 16B) from this zone has phenocrysts of olive to green hornblende (Fig. 4D), brown biotite, and
216 oscillatory-zoned plagioclase (~An ) that reach 5 mm in length. Sparse glomerocrysts consist of
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217 hornblende ± biotite ± relict augite ± titanite ± plagioclase. The groundmass consists of stubby to
218 elongate olive-green hornblende (Fig. 4D), brown biotite, blocky plagioclase with distinct core–
219 rim boundaries (An & An , respectively), interstitial and poikilitic alkali feldspar, and sparse
33 20
220 Fe-Ti oxides.
221 Hornblende was analyzed from one sample of the eHD adjacent to the KCL. This sample
222 (KCL-214; Fig. 4C)) is medium- to coarse-grained granodiorite with hypidiomorphic granular
223 texture and olive to green hornblende that occurs as euhedral crystals to one cm long, as smaller
224 subhedral to anhedral prisms, and as interstitial grains. Some grains show patchy zoning and a
225 few are partly replaced by blue-green actinolitic amphibole. Hornblende encloses apatite,
226 magnetite, and plagioclase and shows minor alteration to chlorite and epidote. The remainder of
227 the sample consists of brown, euhedral, anhedral, and subpoikilitic biotite, euhedral to anhedral
228 plagioclase, and interstitial quartz and microcline. Some plagioclase shows broad oscillatory
229 zoning (~An ) and some has ~An cores and An rims. Euhedral titanite straddles grain
30 40 20
230 boundaries (Fig. 4C), forms inclusions in microcline, and is intergrown with hornblende rims.