Petrology of ferroan alkali-calcic granites: Synorogenic high-temperature melting of undepleted felsic lower crust (Damara orogen, Namibia)

    Petrology of ferroan alkali-calcic granites: Synorogenic high-temperature melting of undepleted felsic lower crust (Damara orogen, Namibia) J. Stammeier, S. Jung, R.L. Romer, J. Berndt, D. Garbe-Sch¨onberg PII: DOI: Reference:

S0024-4937(15)00088-2 doi: 10.1016/j.lithos.2015.03.004 LITHOS 3546

To appear in:

LITHOS

Received date: Revised date: Accepted date:

18 November 2014 25 February 2015 1 March 2015

Please cite this article as: Stammeier, J., Jung, S., Romer, R.L., Berndt, J., GarbeSch¨ onberg, D., Petrology of ferroan alkali-calcic granites: Synorogenic high-temperature melting of undepleted felsic lower crust (Damara orogen, Namibia), LITHOS (2015), doi: 10.1016/j.lithos.2015.03.004

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ACCEPTED MANUSCRIPT Petrology of ferroan alkali-calcic granites: Synorogenic high-temperature melting of

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undepleted felsic lower crust (Damara orogen, Namibia)

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Stammeier, J.1*, Jung, S.1, Romer, R.L.2, Berndt, J.3, Garbe-Schönberg, D.4

Fachbereich Geowissenschaften

Mineralogisch-Petrographisches Institut

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Universität Hamburg

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20146 Hamburg, Germany e-mail: [email protected]

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e-mail: [email protected]

Helmholtz-Zentrum Potsdam

Telegrafenberg

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14473 Potsdam

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Deutsches GeoForschungsZentrum GFZ

e-mail: [email protected]

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Institut für Mineralogie

Universität Münster Corrensstrasse 24 48149 Münster, Germany e-mail: [email protected]

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Institut für Geowissenschaften,

Abteilung Geologie

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Universität Kiel

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Ludewig-Meyn-Strasse 10

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24118 Kiel, Germany e-mail: [email protected]

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*corresponding author

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Abstract

The 556 ± 4 Ma-old Bloedkoppie granite (Central Damara orogen, Namibia) is a

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metaluminous to slightly peraluminous, alkali-calcic to calc-alkalic and ferroan granite. Its composition implies high-temperature, reduced, and anhydrous conditions

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during granite formation. The granite is fractionated, but heterogeneous radiogenic isotope data (87Sr/86Sr(init.): 0.712 to 0.727; Nd(init.): -7.2 to -13.1; 206Pb/204Pb: 17.30-

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17.72; 207Pb/204Pb: 15.54-15.67; 208Pb/204Pb: 37.80-38.23) indicate also that combined assimilation-fractional crystallization processes played an important role in the generation of the granite. Major and trace element compositions and isotope data of the least evolved samples and U-Pb data from zircon cores demonstrate that the source rocks are dominated by ca. 1.95 Ga old felsic orthogneisses from the underlying basement. Zircon saturation temperatures and normative Qz-Ab-Or compositions indicate minimum melting P-T conditions of ca. 860°C at > 5 kbar and < 5 wt.% H2O. The most likely petrogenetic model involves high temperature partial melting of a Paleoproterozoic felsic source in the lower crust ca. 10-20 Ma before the first peak of regional high-temperature metamorphism. Underplating of the lower

ACCEPTED MANUSCRIPT crust by magmas derived from the lithospheric mantle may have provided the heat

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for melting of the undepleted basement to produce reduced and anhydrous melts.

ACCEPTED MANUSCRIPT 1. Introduction

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Granite plutons contain information on processes related to their generation and

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differentiation, including (i) the conditions of melting, (ii) the geochemical and isotopic

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signatures of their sources, (iii) the extent of fractional crystallization processes and (iv) evidence for possible assimilation of wall rock (e.g. Castro, 2013; Chappell et al., 1987; Clemens and Stevens, 2012; DePaolo, 1981). Primary melts of granitic

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composition can form by partial melting of pre-existing crustal rocks (e.g. Clemens et

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al., 1986; Collins et al., 1982; Chappell and White, 2001; King et al., 2001), by differentiation of juvenile material (Frost and Frost, 1997; Loiselle and Wones, 1979), or by the interaction of juvenile melts with older crust (Frost and Frost, 1997). The

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wide range of sources and processes that may be involved in the formation of granites is reflected in the compositional range of granitoid rocks, in particular in their

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trace element abundances and radiogenic isotope compositions. Collisional orogens are generally characterized by widespread high-grade

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metamorphism and major crustal magmatism producing felsic melts. Among these magmatic rocks, early syn-orogenic granites are particularly interesting for unraveling the nature and extent of crustal processes and conditions in the early history of orogens. The development of such granites provides direct insights into the local evolution of an orogen and in general processes of crust-internal material redistribution. The Damara Orogen with its regional exposures of deep crustal metamorphic and plutonic rocks allows to study in detail the spatial, temporal and genetic relation between granite and high-grade metamorphic rocks. In the Damara Orogen, the main interval of igneous activity during the Pan-African orogeny broadly coincides with granulite-facies metamorphism in the deep crust. Therefore, granite formation was inferred to be bound to relatively dry conditions during high-

ACCEPTED MANUSCRIPT temperature melting (McDermott et al., 1996; Jung et al., 1999; Jung and Mezger, 2001).

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Recent work provided new and precise temporal constraints on the igneous and

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metamorphic activity in the orogenic belt. In collisional orogens the production of

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large quantities of granitic melt at dominantly dry conditions requires additional heat input (Clark et al., 2011). This can be generated by raised radiogenic heat production in the crust or heat influx through the base of the crust, either due to lithospheric

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thinning or mantle derived melts that intrude into the lower crust. This work studies

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the generation of early syn-orogenic high-T granites, focusing on the Bloedkoppie granite, which is located within the Okahandja Lineament Zone, in the Central

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2. Geologic setting

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Damara Orogen

The Damara orogen is part of the Neoproterozoic Pan-African mobile belt system in

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southern Africa and is a remnant of the collision of the Congo craton in the north, with the Kalahari craton in the south and the Río de la Plata craton in the west. These collisions formed the NNW-SSE oriented Gariep belt, the N-S oriented Kaoko belt and the NE-SW trending Damara belt (Fig. 1; Miller, 1983; 2008). Based on stratigraphy, metamorphic grade, structure, and age, the Damara belt has been divided into a Northern Zone (NZ), Central Zone (CZ) and a Southern Zone (SZ) (e.g. Miller, 1983). The age of the basement is ca. 2.5 Ga in the Kaokoveld (Seth et al., 1998, Franz et al., 1999) and ca. 1.9 – 2.1 in the CZ and NZ (Jacob et al., 1978, Tegtmeyer and Kröner, 1985) and ca. 1.2 – 1.8 Ga in the Southern Zone (Ziegler and Stoessel, 1993). Basement gneisses are overlain by Neoproterozoic metasedimentary sequences that include deep-water turbidites, passive margin

ACCEPTED MANUSCRIPT carbonate-pelite-quarzite sequences, and molasse deposits. These sediment sequences possibly have been deposited in a intracontinental rift that may have

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developed after ca. 780 – 746 Ma (Jung et al., 2007; Gray, 2008) and was

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subsequently closed from 580 – 550 Ma, eventually culminating in the Damara

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orogeny with crustal thickening and thrusting and emplacement of Pan-African plutonic rocks. Whether the Damara belt originates from a former ensialic basin (e.g. Martin and Porada, 1977; Porada 1989) or an ocean basin with oceanic lithosphere

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followed by subduction (Barnes and Sawyer, 1980; Kasch, 1983) is still a subject of

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discussion. A recent compilation (Miller, 2008) favors a rather narrow (ocean) basin with an exclusive ensialic evolution. The Okahandja Lineament Zone (OLZ), which separates the Central Zone from the Southern Zone, is supposed to represent the

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suture of this Neoproterozoic intracontinental subduction of the Southern Zone beneath the Central Zone (Porada, 1989; Prave, 1996).

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The Central Zone was intruded by large volumes of 570 to 480 Ma-old plutonic rocks that according to Miller (1983) comprise mostly of true granites and only rarely

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include diorites and tonalites/granodiorites. Metamorphism reached increasingly higher grade from east to west, with partial melting and granulite facies conditions at 680–750°C and 5–6 kbar in the coastal area (Jung and Mezger, 2003; Jung et al., 2009; Masberg et al., 1992; Jung et al., 1998). This metamorphism in the Central Zone lasted from 540 to 470 Ma, with a peak at around 520-510 Ma (Jung 2000; Jung and Mezger 2001; 2003). In the Southern Zone, metamorphic conditions increased from south to north reaching up to ca. 8 kbar and ca. 600°C close to the OLZ (Hartmann et al., 1983, Kasch, 1983).

3. Field relations and petrography

ACCEPTED MANUSCRIPT The Bloedkoppie granite is situated within the Okahandja Lineament Zone and intrudes into the Tinkas schists (Fig. 2). The pluton appears homogeneous with local

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inclusions of Tinkas schists. The Bloedkoppie granite has a fine to medium grained

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fabric. The common mineral assemblage is quartz, plagioclase, K-feldspar

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(microcline), biotite, and rare muscovite. Myrmekite is present in most samples. Plagioclase shows polysynthetic twinning. In some samples, K-feldspar is altered. Subparallel mineral inclusions of muscovite are observed in K-feldspar pointing to

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incorporation of muscovite during growth of the K-feldspar phenocryst. There are two

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mineralogically distinct suites. One suite is characterized by Fe-rich biotite and rare muscovite. The other suite comprises samples with Mg-rich biotite, euhedral to anhedral titanite, and rare amphibole. Zircon, apatite and Fe-Ti-oxides are similarly

4. Results

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distributed among the two suites and are present in most samples.

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4.1. U-Pb dating

The results of U-Pb zircon dating of sample Bl02 are presented in Appendix A. The analyzed crystals are euhedral with no visible inclusions. Two crystals have inherited cores. The majority of the zircon U–Pb data is concordant with apparent 206Pb/238U ages ranging between 538 and 580 Ma. Plotting the data in a histogram, however, demonstrates that some concordant zircon crystals have a higher

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Pb/238U age,

possibly indicating that the concordant zircon population does not consist of a single population. Omitting these zircon crystals from the age calculation yields an age of 556 ± 4 Ma (Fig. 5 a). There are a few concordant and discordant inherited crystals that have 207Pb/206Pb ages in the range 1950 to 1960 Ma (Fig. 5 b) and there are a

ACCEPTED MANUSCRIPT number of discordant zircon grains that straddle in the 206Pb/238U vs. 207Pb/235U diagram along a discordia between 550 Ma and the origin of the diagram (Fig. 5 c).

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The 555 Ma age is interpreted to reflect the age of intrusion and falls in the

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same age range as other intrusive rocks of the OLZ, as for instance the Gawib

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granodiorites (Osterhus et al., 2014) and the quartz diorites at Goas-Okongava (Jung et al., 2002) and Palmental (Jung et al., 2015). The time of intrusion predates the onset of regional high-grade metamorphism by ca. 15 m.y. The U-Pb ages of ca. 2.0

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Ga of inherited zircon from the Bloedkoppie granite have also been observed in other

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syn-orogenic Damaran granites (1.7 to 2.1 Ga) and may represent the minimum age

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4.2. Major and trace elements

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of the underlying basement.

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Major and trace element compositions are shown in Appendix B. The Bloedkoppie granite has SiO2 contents ranging from 69.3 to 75.5 wt.%, is ferroan (Fe-number: 0.8-

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0.9; Frost et al., 2001) and metaluminous to slightly peraluminous (alumina saturation index, ASI: 0.90 to 1.06). The granite shows a trend from alkali-calcic towards calcalkalic at increasing SiO2 contents. The granite samples have moderate Al2O3 (15.912.9 wt.%) and CaO (1.86-0.57 wt.%) contents. Na2O (4.02-2.81 wt.%) and K2O (4.05-6.61 wt.%) concentrations show only little variation, with K2O tending towards slightly higher abundances and Na2O towards slightly lower abundances with increasing SiO2 concentrations. Abundances of Fe2O3 (total) (4.87-0.85 wt.%), MgO (0.71-0.09 wt.%) and TiO2 (0.56-0.06 wt.%) are generally low. The contents of Ba and Rb are moderate to high (Ba: 725-127 ppm; Rb: 340-221 ppm), whereas the Sr contents are comparably low (Sr: 148-54 ppm). Correspondingly, ratios of Ba/Sr (4.92.4), Rb/Ba (0.3-1.9) and Rb/Sr (1.5-4.5) show a relative enrichment of Ba to Rb and

ACCEPTED MANUSCRIPT Sr and further an enrichment of Rb relative to Sr. Rare Earth Element (REE) abundances show LREE enrichment relative to HREE with LaN abundances ranging

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from 688-30 and LuN abundances from 30-10. Both the LaN/SmN ratios (1.6-4.3) and

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the GdN/YbN ratios (0.9-2.8) are high (Fig. 6). Samples have either positive or

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negative Eu-anomalies (Eu/ √(Sm*Gd): 0.28-1.24), in which the positive Eu-anomaly

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4.3. Sr, Nd and Pb isotopic composition

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is more pronounced in the REE-depleted samples.

The results of the Sr, Nd and Pb isotope analyses are listed in Appendix C. The initial εNd values are rather unradiogenic with values ranging from –7.2 to –13.1. The initial Sr/86Sr values range from 0.7119 to 0.7274. Although there is a general tendency

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that samples with lower εNd values also have higher 87Sr/86Sr values, the data do not

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show a discernible correlation, which would be obtained for a simple two-component system. Instead, most samples define a cluster with εNd values around –10.7 (Fig.

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7). The samples from the Bloedkoppie granite have markedly higher initial 87Sr/86Sr values at comparable εNd than broadly coeval quartz diorites from the Central Zone and have lower εNd values than the Neoproterozoic metasedimentary rocks (Fig. 7). The Sr-Nd isotopic composition of samples from the Bloedkoppie granite falls in the field defined by basement-derived granites occurring in the Central Zone (Fig. 7). Thus, the source of the granitic melts of the Bloedkoppie granite is likely to have a similar or the same source as other granites in the Central Zone. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios range from 17.30 to 17.72; 15.54 to 15.67, and 37.80 to 38.23, respectively, and fall to the low-radiogenic end of the field defined by basement-derived granites (Fig. 8). All samples are characterized by high 207Pb/204Pb at a given 206Pb/204Pb, which is typical for involvement of old

ACCEPTED MANUSCRIPT crustal rocks in their source (e.g. Zartman and Doe, 1981). High 208Pb/204Pb ratios at a given 206Pb/204Pb indicate that these source rocks most likely experienced an

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ancient high-grade metamorphism that caused a relative depletion of U relative to Th

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in the source region.

5. Discussion

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5.1. Age constraints

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Age constraints for the early Paleozoic orogenic processes in the Damara orogen include Rb-Sr ages, which were largely produced in the 1970ies and 1980ies, and UPb ages that mostly originate from the 1990ies and later. The age constraints are

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highly problematic as the older Rb-Sr ages in many cases do not agree with U-Pb ages obtained on the same samples, but many tectonic models have been

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constructed using both Rb-Sr and U-Pb ages of metamorphic and magmatic rocks. As a result the time scale of processes may have been distorted. On the one hand

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processes that actually were coeval may have been assigned to two different events and on the other hand two different processes may have been lumped into a single event. To illustrate this point, Table 1 shows a compilation of Rb-Sr and U-Pb age data obtained on the same rocks in the Damara orogen. To avoid the problem of distorted time scales, we use only U-Pb ages to constrain the various processes and ignore Rb-Sr age data, no matter if they fit into the general time frame or not. Fractional crystallization does not affect the Sr isotopic composition, but the Rb/Sr ratio. The larger the spread of the Rb/Sr ratio in the sample suite used for dating, the more robust is the regression line and, therefore, the better constrained is the slope of the isochron and the age derived from the isochron. However, most granitic systems do not evolve by fractional crystallization alone, but also involve

ACCEPTED MANUSCRIPT assimilation of wall rocks, which may introduce more or less radiogenic Sr into the system. Such a heterogeneous Sr isotopic composition, however, may result in

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inaccurate ages (Romer, 1994).

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The age difference between Rb-Sr WR and U-Pb zircon ages has traditionally

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been explained by resetting of the Rb-Sr. There are two points to these explanations: (i) Rb-Sr ages that are older than the U-Pb zircon ages are not covered by this explanation, but may be due to heterogeneous initial Sr isotopic compsoitions

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(Romer, 1994), and (ii) resetting of the Rb-Sr system, which is scale dependent as

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demonstrated by Roddick and Compston (1977). Calculation of an Rb-Sr age using the samples from this study would give an age of ca. 624 Ma, which is significantly older than the published Rb-Sr age of ca. 509 Ma from Downing and Coward (1981)

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or above presented U-Pb age (Fig. 8). Examples where the Rb-Sr isochron age is older than the age obtained from U-Pb

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data obtained on accessory minerals haves been observed in the Oas-Lofdal syenites in the Northern Zone (compare data from Jung et al., 2007 with Kröner,

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1982) and the Palmental complex (compare data from Jung et al., 2015 with Kröner, 1982) In view of these major uncertainties associated with the data published by Downing and Coward (1981) we ascribe the apparently older Rb-Sr age of the Bloedkoppie granite obtained in this study to the assimilation of wall-rocks with high 87

Sr/86Sr during AFC processes (see below).

5.2. The role of fractional crystallization and assimilation-fractional crystallization processes

The chemical composition of the Bloedkoppie granite reflects fractional crystallization of plagioclase, biotite, apatite, zircon, and possibly magnetite (Fig. 3). K2O shows

ACCEPTED MANUSCRIPT only little variation consistent with some minor accumulation of K-feldspar. Modelling the fractionation process using trace element abundances and internally consistent

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partition coefficients (Bacon and Druitt, 1988), indicates a maximum extent of 35-40%

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fractionation with 55% biotite and 45% plagioclase. Fractionation of plagioclase

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accounts for the correlated decrease of Eu and Sr in the more evolved samples (Fig. 4).

Chemical variability in granites can be achieved by many different processes

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that include amongst others assimilation of wall rock coupled with fractional

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crystallization (AFC; DePaolo, 1981), progressive partial melting (e.g. Clemens and Watkins, 2001) and crystal entrainment and unmixing (Chappell et al., 1987; for a full discussion see Clemens and Stevens, 2012). In addition, Castro (2013) has shown

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that chemical composition of granites may represent incompletely separated melts from their solid residues. These granites may be called cumulative granites (Castro,

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2013). Most of the samples investigated in this study seem to follow a simple igneous fractionation path (Fig. 3), however the most primitive sample of the Bloedkoppie

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granite is geochemically distinct and has the highest abundance of Al2O3, K2O, CaO, MgO, Fe2O3 and TiO2 and the lowest abundance of Na2O and K2O. These features are inconsistent with accumulation of plagioclase (high CaO but low Na2O) and also inconsistent with accumulation of K-feldspar (high Al2O3 but low K2O). High abundance of Fe2O3, MgO and TiO2 are principally compatible with accumulation of mica. However, this sample has the lowest Ba, lowest Sr and lowest Rb contents which is incompatible with accumulation of feldspar and mica. In addition, this sample has the highest REE contents and the most pronounced Eu anomaly which indicates rather removal of feldspar than accumulation. The next most primitive sample has high Al2O3, K2O but low CaO, Fe2O3, MgO and low TiO2. These features would be consistent with accumulation of K-feldspar and removal of plagioclase and mica. In

ACCEPTED MANUSCRIPT conclusion, although we can not fully rule out that the most primitive samples represent cumulative granites (cf. Castro, 2013) the geochemical features are

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inconsistent with a simple accumulation/melt separation history. Excluding the low

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silica sample with low CaO, Fe2O3, MgO and TiO2 abundances, the results of Zr-

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saturation temperature calculations, Qz-Ab-Or systematics and FC and AFC modelling are almost similar between the most primitive sample and the primitive sample that defines the fractionating sequence; this observation indicates that the

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conclusions raised are still valid.

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The Nd and Pb isotopic compositions are heterogeneous. Initial 87Sr/86Sr ratios show some correlation with SiO2 abundances with a trend towards more radiogenic values with increasing SiO2, which indicates the assimilation of old crust

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characterized by radiogenic Sr isotopic compositions. Assimilation of old crustal material is in line with the presence of ca. 1.95 Ga old inherited zircon in the granite.

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However, the 1.95 Ga old zircon may equally represent the age of the source. Unfortunately it is not possible to place precise constraints on each possibility. Nd

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values show no clear systematic variation with respect to major element chemistry. Estimating the degree of contamination by AFC processes depends critically on the model specific variables and the composition of the contaminant. From the trend of isotopic data from the Bloedkoppie granite, it is apparent that the contaminant should have rather radiogenic Sr but unradiogenic Nd isotopic compositions. Hence, the minimum composition of the contaminant is εNd < –15 and Sri >0.750. The ratio of mass assimilated to mass fractionated is set to r=0.5 and corresponds to F~0.7 (remaining fraction of the original melt). This is equivalent to assimilation of ca. 30% of ancient crustal material. For a more extreme isotope composition of the contaminant and/or higher r-values the amount of assimilated material is even smaller. The contaminant falls in the lower range of basement-

ACCEPTED MANUSCRIPT derived granites and it is reasonable to assume that the underlying Proterozoic basement served as a contaminant, provided that no intracrustal fractionation of

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Sm/Nd had occurred.

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5.3. Constraints on the source of the granite

Leucocratic granitic melts are likely derived from melting of metapelites,

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metagreywackes or orthogneisses under water-undersaturated conditions (e.g. Miller,

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1985; McDermott et al., 1996; Holtz and Barbey, 1991; Holtz and Johannes, 1991; Castro et al., 2000; Villaseca et al., 1998; Jung et al., 2009, Montel and Vielzeuf, 1997).

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The Bloedkoppie granite is metaluminous to weakly peraluminous. Major element concentrations of the most primitive samples (SiO2 ≤ 72 wt.%) are low CaO

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(~1.8 wt.%), low Al2O3 (~14 wt.%), low Fe2O3 (~2.6 wt.%) and moderate Na2O (~3.5 wt.%). The most primitive samples of the Bloedkoppie granite are enriched in Ba

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(271-538 ppm) and Rb (247-340 ppm) relative to Sr (96-109 ppm), leading to high Rb/Sr (1.5-4.5) and Ba/Sr (2.5-4.5) ratios. The enrichment of Ba and Rb relative to Sr is in good agreement with the characteristics of either an orthogneiss-dominated (Holtz and Barbey, 1991; Holtz and Johannes, 1991) or a greywacke-dominated source (McDermott et al., 1996). Major element abundances of samples with SiO 2 ≥ 72 wt.% show some overlap with the leucogranites and experimental melts from Holtz and Barbey (1991) and Holtz and Johannes (1991) as well as some experimental melts from Conrad et al. (1988) and Skjerlie and Johnston (1996; Fig. 9). In these experiments orthogneisses were used as starting material with major element abundances of SiO2: 59.0-65.9 wt.%, Al2O3: 15.1-18.6 wt.%, Na2O: 3.1-4.0 wt.%, K2O: 1.6-4.0 wt.%, CaO: 0.5-5.3 wt.% and Fe2O3: 4.3-9.2 wt.%. A sample from

ACCEPTED MANUSCRIPT the Damara basement gneisses close to the Gawib pluton (Osterhus et al. 2014) fits well into this range (SiO2: 61.1 wt.%, Al2O3: 15.6 wt.%, Na2O: 2.9 wt.%, K2O: 7.2

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wt.%, CaO: 1.6 wt.% and Fe2O3: 7.2 wt.%). For comparison only results with similar

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melting conditions as proposed for the Bloedkoppie granite were considered

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(~875°C, ≥4-5 kbar, see chapter 5.4). All experiments were run under waterundersaturated conditions, i.e. ≤5 wt.% H2O. Melt fraction and chemical composition of the resulting melts is strongly dependent on water content and temperature.

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However, a general trend of increasing CaO, Fe2O3, MgO and NaO2 but decreasing

temperature can be recognized.

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SiO2 and K2O contents with increasing water content (i.e. melt fraction) and/or

In the Central Zone the stratigraphy beneath the Tinkas formation comprises

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the Neoproterozoic Karibib, Chuos, Rössing, Khan and Etusis formations in which greywackes are less abundant. Isotopic compositions for metasedimentary

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sequences of the Khan and the Etusis formation show no overlap with the Bloedkoppie granite (Fig. 7, 8). It is therefore concluded that greywackes are an

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unlikely source for the Bloedkoppie granite whereas orthogneisses from the underlying Palaeoproterozoic basement are more probable source rocks. The chemical and isotopic composition of the Bloedkoppie granites is the result of combined fractional crystallization and assimilation processes during magma evolution. Only the chemical and isotope data of the most primitive samples may reflect the composition of the source rocks. The chemically most primitive samples of the Bloedkoppie granite, however, are not those with the most radiogenic εNd values and unradiogenic 87Sr/86Sr ratios. The chemically most primitive samples (SiO2: 69.371.1 wt.%) have initial εNd values of –10.3 to –12.6 and initial 87Sr/86Sr ratios of 0.7189 to 0.7216. Comparing these samples with the compositions of basementderived granites and metasedimentary rocks from the Central Zone demonstrates

ACCEPTED MANUSCRIPT that the Bloedkoppie granite falls into the field of the basement-derived granites (Fig. 7).

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Lead isotope ratios from leached feldspar reflect the initial Pb isotope

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composition of the granite. 207Pb/204Pb and 208Pb/204Pb ratios show some variation at

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almost constant 206Pb/204Pb (Fig. 8). In the 207Pb/204Pb vs. 206Pb/204Pb diagram, the samples scatter around the upper crustal evolution curve of Zartman and Doe (1981), whereas in a plot of

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Pb/204Pb vs. 206Pb/204Pb the samples fall between the

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evolution curves of lower crust and upper crust, orogenic crust, and the mantle (Fig.

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8). The high 207Pb/204Pb values imply that the Pb is dominantly derived from an old crustal source, which is in line with the ca. 1.95 Ga old inherited zircon. The relatively high 208Pb/204Pb values reflect a high average Th/U ratio in the source rocks. Such

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high Th/U values are characteristic for high-grade metamorphic rocks (e.g. granulites) and imply that the crustal source rocks had experienced an older, e.g.

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Proterozoic or Archean, high-grade metamorphism. Derivation of the granites from ancient felsic lower crustal material is also supported by the DM Nd model ages of

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1.7-2.4 Ga. Plots of 206Pb/204Pb and 207Pb/204Pb against εNd or 87Sr/86Sr show no correlation.

Paleoproterozoic orthogneisses occur in the Abbabis basement inlier in the CZ. This basement inlier is intruded by several basement-derived granites. Isotope data for clearly identified basement rocks are not available. However, a sample taken from presumably undisturbed basement gneisses close to the Gawib pluton yielded Sr-Nd isotope compositions that plot well in the range of the basement derived granites (Osterhus et al., 2014).

5.4. Constraints on temperature and pressure conditions during granite formation and level of emplacement

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Comparison of experimental products with natural granites (i.e. Holtz and Barbey,

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1991; Holtz and Johannes, 1991) is only feasible if the melting conditions used

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during the experiments are comparable to the melting conditions of the respective

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granite. Estimates on temperature and pressure during magma generation can be made under the assumption that the most primitive samples of a given granite suite are close to primary melts. Melting temperature can be estimated using zircon

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saturation (Watson and Harrison, 1983) and Al-Ti thermometry (Jung and Pfänder,

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2007). Zirconium contents decrease with progressing differentiation indicating crystallization and fractionation of zircon. In the Bloedkoppie granite, inherited grains are rare, but indicate that the magma was saturated in Zr (Miller et al., 2003).

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Disregarding the most primitive sample with unusual high TiO2 but low Al2O3 contents, Al-Ti thermometry yields a temperature of ca. 875°C (Fig. 10). Zircon

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saturation thermometry yields a temperature of ca. 883°C. For granitic melts, estimates of pressures during formation and emplacement

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of the magmas can be made using Qz-Ab-Or systematics. It has been noted that lower water contents or increasing pressure enlarge the stability field of quartz (e.g. Nekvasil and Burnham, 1987). It is likely that lower crustal melting at ca. 550 Ma occurred under water-undersaturated conditions due to earlier high grade metamorphism. Comparison of the samples with the cotectic lines for waterundersaturated conditions of 5 wt. % H2O (Fig. 11; Long et al., 1986) suggests a pressure of ≥ 5 kbar for the most primitive samples. More evolved samples indicate pressures < 5 kbar, probably still at water-undersaturated conditions. The higher pressures are considered to represent the pressures during magma generation and probably segregation. This pressure estimate corresponds to a depth of magma generation of at least 23 km. Emplacement of the granite occurred at higher levels,

ACCEPTED MANUSCRIPT which is confirmed by pressure-temperature estimates for the country rocks which

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are ca. 4-5 kbar and 650°C corresponding to a depth of ca. 15-19 km (Jacob, 1974).

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5.5. Implications for the generation of early syn-orogenic granites in the Damara

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orogen

Between ca. 570 and 480 Ma the mid-crust of the Central Zone of the Damara

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orogen was intruded by numerous high-T granites. The intrusion of early syn-

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orogenic granites at ca. 550 Ma including the Bloedkoppie granite was broadly coeval with the first high temperature regional metamorphic peak with temperatures up to 680-750°C at pressures of 5-6 kbar (Jung and Mezger, 2003). Temperature and

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pressure estimates on the Bloedkoppie granite and other early- to syn-orogenic granites indicate that the lower crust was already hot at the time of the inferred onset

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of the first metamorphic peak (e.g. Ostendorf et al., 2014; Osterhus et al., 2014; Jung et al., 2015). Temperature-pressure estimates on high-grade metamorphic rocks from

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the Central Zone and temperature estimates on early syn-orogenic granites imply a geothermal gradient of ca. ≥30°C/km in mid- to lower crustal regions of the Damara orogen. Previous investigations by Haack et al. (1983) have shown high radioactive heat production (2.5–8.8 μWm−3) in meta-igneous units of the Damara orogen. Clark et al. (2011) argue that raised heat production of at least 3.5 μWm−3 are necessary to achieve a temperature sufficient for large-scale partial melting in lower crustal units. However, the temperature peak due to internal heat production will occur ca. 40-60 Ma after crustal thickening (Clark et al., 2011). This view is consistent with the observation that the main period of crustal melting in the Damara Orogen producing mainly S-type and minor A-type granites (but not the Bloedkoppie granite and others in the OLZ) occurred at 520-500 Ma, i.e. ca. ~20–40 Ma after the first peak of

ACCEPTED MANUSCRIPT metamorphism which occurred at ca. 540 Ma (Jung and Hellebrand, 2006; Jung and Mezger, 2003; Jung et al., 2000; Longridge et al., 2011) and 30-50 Ma after initiation

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of thrusting. While this excess temperature caused by internal heat production might

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explain the production of large volumes of syn- to late-orogenic granites, melting of

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lower crustal units contemporaneous with crustal thickening most likely requires an additional thermal input. In this case, high geothermal gradients may be produced through heat from intruding magmas as well as thermal conduction from the

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underlying lithospheric mantle (Annen et al., 2006; Annen and Sparks, 2002). Annen et al. (2006) have shown that the emplacement of mafic, mantle-derived magmas

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into the lower crust resulting in ―hot zones‖ may lead to the generation of intermediate and felsic melts. ―Hot zones‖ provide heat that facilitates partial melting

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of crustal rocks, or alternatively lead to partial crystallization of mafic, mantle-derived magmas that produce residual H2O-rich melts. This model was developed to explain

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the genesis of subduction-related felsic magmas, but may also be applicable to other tectonic settings where high-T granites occur. The model incorporates mafic mantle-

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derived magmas that intrude deep crustal levels and induce local melting, assimilate and mix with crustal rocks, and thus, produce a zone of crystal-melt mush that undergo dynamic homogenization as well as periodic magma recharge (Hildreth and Moorbath, 1988). The presence of mantle material which is essential for this model is supported by early syn-orogenic, ca. 545 Ma-old, presumably lithospheric mantlederived quartz diorites from the Palmental complex that intruded into the OLZ (Jung et al., 2015) and is also consistent with recent views on possible subduction zone processes in the Damara orogen (Meneghini et al., 2014).

The Bloedkoppie granite

is a ferroan, alkali-calcic and metaluminous granite (Fig. 12 a, b). Highly fractionated samples are slightly peraluminous. Granites with these chemical characteristics commonly form in intracratonic settings under reduced and anhydrous conditions

ACCEPTED MANUSCRIPT (Frost et al., 1999). Common to these granites are their geochemical features that resemble the composition of A-type granites, i.e. enrichment of highly incompatible

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trace elements such as Zr, Nb, Y, and Ce. A petrogenetic model commonly used to

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explain the formation of such granites is the differentiation of mafic mantle-derived

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melts (Frost et al., 2001; compare reference fields in Fig. 12). However, such a model is unlikely for the Bloedkoppie granite as isotope constraints rule out a connection with mantle derived melts such as the coeval quartz diorites (Jung et al., 2015).

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The Wangrah granite, an A-type granite from Australia, represents another

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example of a formation of ferroan, alkali-calcic and metaluminous granite (King et al., 2001). This granite is geochemically very similar to the Bloedkoppie granite and the inferred conditions during granite formation compare well (Wangrah granite: 4-2 kbar;

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>830°C; 2-5 wt.% H2O; Bloedkoppie granite: > 5-4 kbar; ca. 875oC, ≤ 5 wt.% H2O). The Wangrah granite is interpreted to have formed by partial melting of undepleted

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lower crust.

We favor a single-stage petrogenetic scheme where the Bloedkoppie magmas

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were produced by high-temperature partial melting of quartzo-feldspathic crustal rocks from the Pre-Pan African basement. The relatively refractory nature of the source rocks may have been due to limited H2O content in that source. The important point here is that anhydrous melting of an undepleted felsic crust can produce granitic melts with geochemical characteristics typical for some A-type granites (e.g. King et al., 2001) In conclusion, underplating of the lower crust by mafic magmas probably initiated partial melting of felsic mid- to lower crustal units producing anhydrous and reduced melts that are represented by the Bloedkoppie granite. This view is supported by the presence of quartz diorites from the nearby, coeval Palmental complex that were likely generated by underplating of lithospheric mantle-derived

ACCEPTED MANUSCRIPT magmas. Hence, high-temperature partial melting in the lower crust prior to the first peak of metamorphism is likely facilitated by heat contribution from the lithospheric

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upper mantle.

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6. Summary

The Bloedkoppie pluton (Damara orogen, Namibia) is a metaluminous to slightly

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peraluminous, alkali-calcic to calc-alkalic, ferroan granite that intrudes at ca. 550 Ma

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into mid-crustal levels. The samples have major element abundances that range from 69.3 - 75.5 wt.% SiO2, 1.86 - 0.57 wt.% CaO, 4.02 - 2.81 Na2O and 4.05-6.61 wt.% K2O and have high Rb/Sr (1.5-4.5) and Ba/Sr (2.5-4.5) ratios. The samples show

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enrichment in LREE relative to HREE and a negative Eu-anomaly. Major and trace element variations are compatible with fractional crystallization of plagioclase, biotite,

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apatite, zircon and most likely magnetite during differentiation. Modelling crystal fractionation processes suggests moderate to high extents of fractional

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crystallization. The Nd and Sr isotope compositions and the radiogenic Pb isotope compositions are consistent with derivation of the Bloedkoppie granite from Proterozoic basement rocks. In addition, rare restitic zircon with concordant ages of ca. 2 Ga and the range of DM Nd model ages between 1.7 and 2.4 Ga also indicate involvement of an ancient crustal component. Granite formation occurred in the mid to lower crust at > 5 kbar and ca. 870°C. Experimental partial melting of orthogneisses conducted at 800°C and 3-5 kbar under water-undersaturated conditions (Holtz and Johannes, 1991) yielded granitic melts with geochemical characteristics similar to the Bloedkoppie samples. High Zr+Nb+Ce+Y contents of the least felsic samples are similar to some A-type granites from Australia and indicate, in this case derivation from an undepleted source. Thus,

ACCEPTED MANUSCRIPT it is possible that the Bloedkoppie granite was generated under anhydrous and reduced conditions by high temperature partial melting of ancient undepleted meta-

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igneous lithologies and underwent coupled assimilation-fractional crystallization

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processes during ascent through the crust. The intrusion of the Bloedkoppie granite

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predates high temperature metamorphic conditions by ca. 15 Ma and high temperature conditions in the lower crust are aided by intra- and under-plating of

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lithospheric mantle magmas in the lower crust.

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Acknowledgements

The authors are very grateful to E. Thun and P. Stutz for support in the laboratory and

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preparation of thin sections at the Universität Hamburg. We are thankful to M. Trogisch and B. Schmitte of the Westfälische Wilhelms-Universität Münster for U-Pb

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analysis of zircon by LA-ICP-MS. U. Westernströer and S. Nordstad of the Christian

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Albrechts Universität Kiel are thanked for measurements of trace elements.

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fluid-flow and fluid immiscibility in the Bufa del Diente aureole, NE-Mexico:

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Evidence from radiographies and Li, B, Sr, Nd, and Pb isotope systematics. Contribution to Mineralogy and Petrology 149, 400–429.

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Seth, B., Kröner, A., Mezger, K., Nemchin, A. A., Pidgeon, R.T., Okrusch, M., 1998.

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Archaean to Neoproterozoic magmatic events in the Kaoko belt of NW Namibia and their geodynamic significance. Precambrian Research 92, 341–363.

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Sheraton, J.W., Black, L.P., Tindle, A.G., 1992. Petrogenesis of plutonic rocks in a Proterozoic granulite-facies terrane—the Bunger Hills, East Antarctica. Chemical

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Skjerlie, K.P., Johnston, A.D., 1996. Vapour-absent melting from 10 to 20 kbar of crustal rocks that contain multiple hydrous phases: implications for anatexis in the deep to very deep continental crust and active continental margins. Journal of Petrology 37, 661–691. Tegtmeyer, A., Kröner, A., 1985. U-Pb zircon ages for granitoid genesis in northern Namibia and their significance for Proterozoic crustal evolution in southwestern Africa. Precambrian Research 28, 311–326. Villaseca, C., Barbero, L., Rogers, G., 1998. Crustal origin of Hercynian peraluminous granitic batholiths of Central Spain: petrological, geochemical and isotopic (Sr, Nd) constraints. Lithos 43, 55–79.

ACCEPTED MANUSCRIPT Watson, E.B., Harrison, T.M., 1983. Zircon saturation revisited: temperature and composition effects in a variety of crustal magma types. Earth and Planetary

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Science Letters 64, 295–304.

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Whalen, J.B., Currie, K.L., Chappell, B.W., 1987. A-type granites: geochemical

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characteristics, discrimination and petrogenesis. Contributions to Mineralogy Petrology 95, 407–419.

Zartman, R.E., Doe, B.R., 1981. Plumbotectonics—the model. Tectonophysics 75,

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135–162.

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Ziegler, U., Stoessel, G., 1993. Age determinations in the Rehoboth Basement Inlier, Namibia. Geological Survey of Namibia, Ministry of Mines and Energy Memoir 14,

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1–106.

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Figure captions

Fig.1. Generalized geological map showing the study area within the Central Zone of

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the Damara orogen, Namibia. NZ: Northern Zone; CZ: Central Zone; SZ: Southern Zone. Distribution of regional metamorphic isograds within the southern and central Damara orogen according to Hartmann et al. (1983). Isograds: (1) biotite-in; (2) garnet-in; (3) staurolite-in; (4) kyanite-in; (5) cordierite-in; (6) andalusite ↔ sillimanite, (7) sillimanite-in according to staurolite breakdown; (8) partial melting as a result of muscovite+plagioclase+quartz+H2O ↔ melt+sillimanite; (9) K-feldspar+cordierite-in; (10) partial melting as a result of biotite+K-feldspar+plagioclase+quartz+cordierite ↔ melt+garnet. The Okahandja Lineament corresponds to the southern one of the two dashed lines. Abbreviations in inset map: KZ: Kaoko Zone; NP: Northern Platform; nCZ: northern Central Zone; sCZ: southern Central Zone; SMZ: Southern Margin Zone; S: Swakopmund; W: Walvisbaai; Wh: Windhoek.

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Fig. 2. Geological map of the Bloedkoppie area (simplified after Geological Survey of

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Namibia, 1995).

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Fig. 3. Major and trace element variation diagrams for the Bloedkoppie granite.

Fig. 4. Plot of Eu (ppm) vs. Sr (ppm) concentrations. The positive correlation is

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indicative for fractional crystallization of plagioclase.

Fig. 5. U-Pb Concordia diagram showing U-Pb zircon analyses of sample Bl02 from the Bloedkoppie granite: (a) concordant data points cluster around ca. 550 Ma; Inset:

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Apparent 206Pb/238U ages show a Gaussian distribution with a best fit of ca. 550 Ma; (b) concordant and discordant restitic zircon data indicate an age of ca. 1950 Ma; (c)

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concordant and discordant zircon data with Phanerozoic apparent 206Pb/238U ages.

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Fig. 6. Chondrite-normalized Rare Earth Element plots for the Bloedkoppie granites. Chondrite values from McDonough and Sun (1995).

Fig. 7. Initial εNd vs. initial 87Sr/86Sr diagram for the granite samples (this work, McDermott et al., 1996). Fields represent isotope data of basement-derived intrusive rocks from the Central Zone (Jung et al., 2003; Osterhus et al., 2014; Ostendorf et al., 2014), metasedimentary rocks (McDermott et al., 1990; Jung, 2005), and quartz diorites (Jung et al., 2002; Jung et al., 2015).

Fig. 8. Plot of (a) 208Pb/204Pb and (b) 207Pb/204Pb vs 206Pb/204Pb isotope ratios of leached K-feldspar from the Bloedkoppie granite. The curves represents the average

ACCEPTED MANUSCRIPT Pb growth curves for lower crust (LC), orogenic crust (OC), upper crust (UC), and the mantle (M) (Zartmann and Doe, 1981). Fields represent isotope data of basement-

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derived intrusive rocks (Jung et al., 2003; Osterhus et al., 2014; Ostendorf et al.,

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2014), metasedimentary rocks (Jung, 2005), and quartz diorites (Jung et al., 2002)

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from the Central Zone.

Fig. 9. Plot of selected major element abundances of the Bloedkoppie granite, data

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from experimental melting studies of orthogneisses and orthogneiss-derived

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leucogranites. Abbreviations: H&B: Holtz and Barbey, 1991; H&J: Holtz and Johannes, 1991; C et al: Conrad et al., 1988; S&J: Skjerlie and Johnston, 1996.)

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Fig. 10. Calculated temperatures from zircon saturation thermometry (Watson and Harrison, 1983) and Al-Ti thermometry (Jung and Pfänder, 2007). The calibration of

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the Al-Ti thermometer bases on Skjerlie and Johnston (1996), Conrad et al. (1988),

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and Holtz and Johannes (1991).

Fig. 11. Normative Qz-Ab-Or composition of the Bloedkoppie granite relative to experimental H2O-saturated minimum melt composition (dashed lines: Anderson and Bender, 1989; solid lines: Long et al., 1986)

Fig. 12. (a): FeOtotal/(FeOtotal+MgO) vs. SiO2 diagram for samples of the Bloedkoppie granite: Fe* line after Frost et al. (2001). (b): MALI (=Na2O+K2O-CaO; modified alkali lime index; Frost et al., 2001) vs. SiO2 abundances. MALI boundaries after Frost et al. (2001). (c): Plot of combined Zr+Nb+Ce+Y abundances vs. Ga/Al ratios showing some A-type granite suites and Bloedkoppie granite samples. Rectangular field represents the field covered by fractionated felsic granites and unfractionated M-, S-,

ACCEPTED MANUSCRIPT I- type granites (after Whalen et al., 1987). Reference fields show data from the Wangrah granite (King et al., 1997; 2001), the Sherman batholith (Frost et al., 1999)

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typical for alkali-calcic/calc-alkalic, ferroan and metaluminous granites.

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ACCEPTED MANUSCRIPT Table 1: Comparison of ages obtained by Rb-Sr whole rock (WR) analysis and U-Pb analysis for Damaran rocks. Location Rb-Sr age (Ma) U-Pb age (Ma) Lithology 510 ± 9

1

556 ± 4

2

Bloedkoppie:

granite

Okombahe

553 ± 22

village pluton:

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Okombahe farm

517 ± 11

pluton:

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3

571 ± 5

4

granite to

3

576 ± 6

4

granite to

LA-ICP-MS (zircon) 3

522 ± 1 to 529 ± 1

Okombahe WR

granodiorite

5

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479 ± 16

granodiorite

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LA-ICP-MS (zircon)

Baukwab,

leucogranite

Pb-Pb evaporation (zircon)

Okombahe

478 ± 27

reserve North:

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Wilhelm-

459 ± 15

Albrechtstal:

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Tsomtsaub:

≥550

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reserve: 3

508 ± 6

6

leucogranite

ID-TIMS (monazite) 3

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484 ± 2 to 488 ± 3

granite

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Pb-Pb evaporation (zircon)

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541 ± 3

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764 ± 60

8

quartz

LA-ICP-MS (zircon)

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Lofdal:

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LA-ICP-MS (zircon)

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506 ± 6

8

granodiorite

LA-ICP-MS (zircon)

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754 ± 8

WR

diorite

10

and granite nepheline-

U-Pb TIMS (titanite)

normative syenite

Oas:

840±13

9

758 ± 4

WR

10

quartz-

U-Pb TIMS (titanite)

normative syenite

Palmental

756 ± 35

11

545 ± 1

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complex:

749 ± 34

11

U-Pb SHRIMP

diorite

WR 1

2

3

4

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Downing and Coward 1981; this study; Haack et al. 1980; Bergemann et al. 2014; Jung 6 7 8 9 et al. 1998; Paul et al. 2014; Haack et al. 1982; Jung et al. 2014; Hawkesworth et al. 10 11 1983; Jung et al. 2007; Kröner 1982; 12 Jung et al. 2015

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Pan-African granite intrusion in the high-grade central part of the Damara orogen.

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New U-Pb zircon ages of ca. 555 Ma imply early syn-orogenic intrusion.

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Geochemical data from high-T granites imply melting of undepleted, felsic crust.

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New Sr-Nd-Pb isotope data imply ancient crustal sources and constrain AFC

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processes.