Supplementary Material to accompany “Anatomy, Age and Evolution of a Collisional Mountain Belt: the Baltoro granite batholith and Karakoram Metamorphic Complex, Pakistani Karakoram” by Searle, Parrish, Thow, Noble, Phillips and Waters.

Analytical Methods and Description of U-(Th)-Pb data of samples

Analytical Methods
All analytical work was carried out at the NERC Isotope Geosciences Laboratory, Keyworth, UK during the period 1999-2003. Samples were jaw-crushed and disk-milled to <420 mm. A modified Superpanner©, heavy liquid separation with methylene iodide, and a Frantz LB-1 magnetic separator were used to concentrate and separate the various high density minerals. Zircon, thorite, allanite, titanite, pyrochlore and monazite crystals were hand-picked under alcohol and wherever possible only the highest quality crack-free crystals, and in the case of zircon and monazite the most inclusion-free and limpid grains, were selected for analysis. All zircons were air-abraded to improve concordance (Krogh 1982) and all minerals were washed in warm distilled HNO3 (4 M for zircon, 2 M for the other minerals) and H2O prior to dissolution. Samples were spiked with a mixed 205Pb-230Th-233U-235U tracer and dissolved in HF-HNO3 or HCl (Parrish 1987; Parrish et al. 1987). Standard chemical separations followed Krogh (1973) with modifications (Corfu & Noble 1992; Parrish et al. 1987). Blanks were somewhat variable during the course of this work, but varied from 1-6 picograms, generally, with the composition of these blanks being averaged over time and applied with reasonable uncertainties.  TIMS data were obtained by single collector peak jumping (Noble et al. 1993) on a VG 354 mass spectrometer using a wide aperture retarding potential filter (abundance sensitivity of 54 ppb) and Philips ion counting Daly detector or in multidynamic mode using a combination  of Faraday cups for the largest ion beams and the 204Pb measured on the Daly detector.  Th isotope measurements were made using a VG Elemental P54 multicollector ICP-MS using static measurement, with analysis of an in-house 230Th-232Th isotopic standard calibrated by the method outlined in Bowen et al. (1998) for measurement of mass bias.  Ages are calculated using the decay constants recommended by Steiger & Jäger (1977). Isochron and concordia age calculations and plots were generated using Isoplot 3.0 (Ludwig 2003), and the errors on ages are reported here are at the 2σ level. 

Description of data from samples and interpreted age.
Hushe granite; Sample T245
Zircons have characteristic igneous morphology and are generally pink, clear, euhedral, doubly terminated crystals. Zircon fraction Z1 contained four long, pink, clear zircons approximately 100 μm in length. Fraction Z2 contained four slightly longer and thicker, clear doubly terminated crystals. Fractions Z3 and Z4 were single zircon crystals about 100 μm in length, both with melt inclusions that appear to transect the cores. Zircons from all fractions were air-abraded for four to eight hours. Fractions Z1, Z3 and Z4 plot near concordia at ~ 40 Ma (Fig. 7). Fraction Z2 is discordant at ~ 43 Ma and clearly contains an inherited component. The upper intercept of a discordia through Z2 and tied to the weighted mean of the younger zircon fractions is 936 ± 61 Ma. The three younger zircon fractions (Z1, Z3, Z4) that contain crystals with obvious igneous morphologies probably best represent the crystallization age. The error ellipses for these data do not overlap and this suggests that either the errors are underestimated and/or the measurement of 204Pb, and therefore the common Pb correction, may be slightly biased. A Tera-Wasserburg plot intersection with concordia, which is less sensitive to this problem, reveals an age of 40.3 ± 1.2 Ma.  This is broadly consistent with the weighted mean of the 206Pb/238U ages of fractions Z1, Z3 and Z4, 40.2 ± 0.3 Ma.  The fact that subduction-related magmatism was taking place in the Karakoram at ~40 Ma is significant, since all other subduction-related I-type magmatism along the Karakoram and the Kohistan – Ladakh - Gangdese batholith appears to have ended at the time of India-Asia collision ~50 Ma. 

K7 granite; Sample T211
Zircons from T211 were mostly clear, inclusion free crystals although some contained a distinct inclusion-rich core. Fraction Z1 was a single zircon crystal chosen because it was especially clear and considered to be core-free. It was colourless, euhedral, approximately 150 μm long and was air-abraded for 4 hours. One fraction containing two green subhedral to euhedral thorite crystals (TH1) about 100 μm long was also picked.  
Fraction Z1 plots concordantly at around 21 Ma whereas fraction TH1 plots slightly above concordia at around 18 Ma (Fig. 8). The reverse discordance shown by TH1 is considered to be due to excess 206Pb from the decay of 230Th (Schärer 1984), since the Th/U ratio of the fraction is 4.3. The 206Pb/238U age of zircon, 21.2 ± 0.2 Ma could reflect the crystallization age of the sample with the thorite age being more sensitive to Pb loss, a pattern also observed in other Baltoro granite samples dated by Parrish & Tirrul (1989) who observed that thorite dates were 2-3 Ma younger than co-existing zircons. Although there is only one dated crystal of zircon, its concordancy and the fact that it was a single crystal of high quality that was strongly air-abraded suggests that the age is reliable. We interpret the crystallization age to be 21.2 ± 0.2 Ma

Baltoro leucogranite, Paiyu; Sample T308
Zircons picked from all samples of the BPU were generally found to contain inherited cores and plot discordantly so monazites were favoured for dating crystallisation. One zircon fraction (Z1) contained two clear, pink crystals that were air-abraded for four hours. Three monazite fractions were chosen to reflect the variation in the monazite population of the sample. Fraction M1 contained three large (200-300 μm) euhedral green monazites. Fraction M2 contained sixteen smaller (10-50 μm) clear crystals and M3 contained twelve cloudy, green monazite fragments, 20-70 μm in size. 
All three monazite fractions from this sample plot just above concordia at between 20 and 25 Ma (Fig. 7). They define an array that probably represents varying degrees of inheritance of an older Late Cretaceous or early Cenozoic monazite component. The reverse discordancy is interpreted to be due to excess of 206Pb from the decay of 230Th (Schärer 1984). The zircon fraction from this sample plots discordantly at around 45 Ma and is interpreted to contain an inherited component with an igneous rim similar in age to crystallization of monazite. The upper intercept of a discordia plotted through 20 Ma and this fraction suggest the older component was 217 ± 26 Ma in age. We regard the crystallisation age of this sample as best represented by the 207Pb/235U age of the youngest monazite fraction (M3), 19.8 ± 0.5 Ma.

Baltoro granite, Trango-Nameless Towers; Sample T311
Two zircon and four monazite fractions from this rock were analysed. Fraction Z1 contained three pink, clear, euhedral, low aspect ratio, crystals approximately 100 μm in length. Fraction Z2 was a single, clear, colourless crystal 150 μm in length. Both zircon fractions were air-abraded for four hours. Fraction M1 contained eight large, clear, green monazite crystals and fragments between 80 and 100 μm in size. M2 and M3 were both fractions of smaller (< 50 μm) clear monazites and M4 contained 10 large (50-100 μm), cloudy, green, monazite fragments.  
	Four monazite fractions from sample T311 plot above concordia between 11 Ma and 17 Ma (Fig. 7). Three of the fractions (M1, M2, M3) define an array which trends away from concordia at ~17 Ma with the younger fractions being progressively more reversely discordant. Fraction M4 plots at 17 Ma, slightly above the array defined by the three other monazite fractions. This is an unusual pattern of discordance.  Monazites certainly appear younger than is generally found in other BPU samples.  208Pb/232Th ages of monazites M2, M3, and M4 are 15.7 ± 0.3, 15.9 ± 0.1, and 16.7 ± 0.4, respectively.  Except for analysis M2, there is good agreement between these 208Pb/232Th ages and the 207Pb/235U ages of the same analysis.  The discrepancy could be explained by appealing to the poor precision of the 207Pb/206Pb analysis for M2 being both imprecise and inaccurate given the very small (~8 picogram) amount of radiogenic 207Pb in this analysis.  By ignoring the 207Pb/235U age of analysis M2, and taking the weighted mean of the remaining 207Pb/235U and 208Pb/232Th ages of the two youngest analyses M2 and M3, we derive an age of 15.8 ± 0.2 Ma for the most likely crystallization age of M2 and M3.  The slightly older M4 monazite is suggested to be due to slight monazite inheritance. The younger M1 analysis appears analytically satisfactory as it has much more radiogenic Pb than M2.  If slight inheritance is a feature, then the maximum crystallization age would be given by the 207Pb/235U age of analysis M1, 13.9 ± 0.2 Ma. The two zircon fractions from this sample plot discordantly opposite ~80 Ma (Z2) and ~800 Ma (Z1) and clearly reflect a varied age of the inherited component and are therefore not useful for constraining the crystallisation age of the BPU. The age of the inherited components in Z1 and Z2 are calculated to be 1677 ± 10 Ma and 537 ± 18 Ma anchoring them to ~20 Ma, respectively, suggesting that pre-existing crystals of a number of ages were incorporated into the BPU, as demonstrated by zircon inheritance in previous samples (Parrish & Tirrul 1989).

Baltoro granite, Cathedral-Biale; Sample T320
One zircon fraction (Z1) contained two, clear crystals 100-150 μm long, with rectangular cross sections and no inclusions and three monazite fractions were picked. Fraction M1 contained three large (300-400 μm) green crystals that contained inclusions. Fraction M2 contained twelve small clear green monazites and M3 contained nine large (200-300 μm), green, cloudy, green monazite fragments.  
Three monazite fractions from sample T320 plot above concordia at 14 Ma, 16 Ma and 17 Ma (Fig. 7) in a manner similar to sample T311. Two of the fractions (M1, M2) define a line which trends away from concordia, while the third (M3) lies slightly below this array at ~17 Ma. The zircon fraction from this sample, Z1, is discordant opposite ~240 Ma. It clearly contains an inherited component and has an upper intercept age of 1034 ± 10 Ma.  From the monazite data, we infer that the likely age of crystallization of monazite is best estimated to be no older than the youngest 207Pb/235U age of M1 at 13.3 ± 0.3 Ma.  208Pb/232Th ages of these three monazites are all ~16.4 ± 0.2 Ma, including analysis M1.  This complex set of data implies that the age is either 13.3 ± 0.3 Ma or 16.4 ± 0.2 Ma.  The interpretation remains ambiguous. Geologically, this sample is related to T311 for which we have inferred the age to be 13.9 ± 0.2 Ma.   Being guided partly by this fact, an age of 13.3 ± 0.3 Ma is used in the text. 

Bakhor Das leucogranite dyke: Sample T97
Zircons from this sample had varying morphologies, mainly short squat prisms with some long, clear, needle-like zircons. One zircon fraction (Z1) contains three of the longer grains (~100 μm). These were inclusion free and one had a crack running the length of the crystal, which was not diffracted across any internal boundaries, suggesting there was no inherited core. Two allanite fractions (A1, A2) containing eight and sixteen grains respectively, all fragments over 100 μm, were also picked from this rock.  
Allanite fractions (A1, A2) plot slightly below concordia at around 25 Ma (Fig. 7).  The 238U/204Pb-206Pb/204Pb isochron age of the allanites is 24.7 ± 3.6 Ma and the 235U/204Pb-207Pb/204Pb isochron age is 27.6 ± 9.3 Ma (Table S2). The isochron age of 24.7 ± 3.6 Ma is considered to be a reliable maximum age for this sample. However, as these allanites have Th-U ratios of around 10, they may contain up to ~1 Ma of excess 206Pb, from the decay of excess 230Th. The actual age may therefore be slightly younger. Zircon fraction Z1 plots slightly below concordia at around 27 Ma (Figure 7), consistent with a ca. 24-25 Ma crystallisation age with a small amount of inheritance. For the purposes of discussion, an age of 24.7 ± 3.6 Ma is used in the main text.

Masherbrum foliated granite; Sample T258
Zircons were dominantly clear, pink, doubly terminated, euhedral crystals with occasional melt inclusions, and appeared to be igneous in origin.  Three zircon fractions were chosen to reflect the variations within the zircon population. Fraction Z1 contained seven long, thin crystals, 50-100 μm in length, with rectangular cross-sections and some melt inclusions. Fraction Z2 contained seven smaller (< 50 μm), clear, doubly terminated crystals. Fraction Z3 contained four large (~150 μm) low aspect ratio zircons with occasional melting inclusions. All fractions were air-abraded for four hours. 
Three zircon fractions analysed from this sample plot slightly below concordia opposite 152 and 158 Ma (Fig. 8). Fractions Z1 and Z3 are very slightly discordant at around 156 Ma whereas fraction Z2 is younger (152 Ma) and more discordant which demonstrates that inheritance is present in at least this latter analysis. It is likely that small amounts of residual Pb loss is also present.  Fractions Z1 and Z3 could lie on the same Pb loss chord and lack inheritance, which could indicate an original crystallisation age of 180 – 170 Ma. No robust age can be assigned to this sample other than it is likely that it is of Late Jurassic age.  For the purposes of the text, we tentatively use an age range of 180 – 170 Ma based on the upper intercept of a Pb-loss chord through Z1 and Z3 or the mean 207Pb/206Pb ages of these two analyses, for the tentative assignment of crystallization age. Clearly more work would be needed to refine this age.

Masherbrum leucogranite sheet; Sample T263
The monazite population in this sample was fairly uniform and three monazite fractions were picked for analysis. Fraction M1 contains eleven small, subhedral, clear monazites approximately 100 μm in size. Fractions M2 and M3 contains eighteen and seventeen (respectively) subhedral crystals of similar size. Fraction M4 contained 10 large (75-100 μm), cloudy, green crystals with some inclusions and brown staining. 
Fractions M2 and M3 plot just above concordia at ~ 18 Ma (Fig. 8) and are in good agreement with each other. The reverse discordancy exhibited by most analyses is probably due to excess 206Pb from the decay of 230Th (Schärer 1990). Fraction M1 plots slightly below concordia at ~ 22 Ma and this fraction is therefore considered to contain an inherited component. Fraction M4 plots slightly above concordia at 47 Ma. This fraction therefore also contains an inherited component although the age of this component must be considerably younger than that in M1, since it still plots above concordia but is no younger than ~47 Ma. This is possible because a discordia with a relatively young (< 200 Ma) upper intercept would lie virtually parallel to concordia and the effect of 206Pb excess would mean that a fraction plotting along the discordia could still be reversely discordant. This suggests that the older component in M1 may be inherited monazite which first crystallised during pre-collisional metamorphism, the ages of which are known to include 63.3 ± 0.4 Ma and 50-52 Ma in the Hunza Karakoram (Fraser et al, 2001). The interpreted crystallisation age of monazite in this sample is therefore taken to be 17.6 ± 1.1 Ma, the weighted mean of the 207Pb/235U ages of fractions M2 and M3.

Lamprophyre dyke sample T248
Zircon crystals were cloudy and euhedral, suggesting that they had formed from the crystallising dyke, but they were considered too small and too few to constitute a useful analysis. Instead, a single larger (~100 μm), rounded crystal was chosen. This was not abraded because there were too few crystals to replace it had it been lost. The sample also contained numerous, tiny titanite crystals. 
Fraction Z1 plots concordantly at around 33 Ma (Fig. 8) and this is age is interpreted to represent the crystallisation age of this sample. Errors on the ages of this fraction are rather large, as a result of the very low U concentration in the fraction and the large amount of common Pb in these unusual zircons. Although the crystallisation age of this rock is constrained by only one analysis, its concordancy suggests that the concordia age (Ludwig 1998) of 32.8 ± 3.6 Ma is a reasonable estimate.

Karakoram metamorphic complex
Kyanite-grade pelite, Panmah-Braldu confluence; Sample T16
Monazite was separated from the specimen, yielding numerous very small (<40 μm diameter) crystals that were difficult to handle and impossible to measure as single grains by ID-TIMS.  On the basis of study of thin section, they occur within the matrix of the main rock forming minerals primarily, but are widespread. All fractions (M1, M2, M3, M4) plot in good mutual agreement at around 22 ± 0.3 Ma (Fig. 7).  Three fractions (M2, M3, M4) plot slightly above concordia, whereas M1 plots very slightly below. The degree of reverse discordancy is most probably related to the incorporation of 230Th (Schärer 1984), although the Th/U ratios for each are almost identical, probably a result of the averaging effect caused by the large numbers of crystals in each analysis, approximately 100 grains. The good agreement and relative concordancy of the analyses suggests that the results are reliable. The weighted mean of the 235U/207Pb ages from all fractions is 22.0 ± 0.3 Ma, and this is the interpreted age of crystallisation. Fraser et al (2001) obtained a U-Pb age of 28.0 ± 0.5 Ma for monazites from an almost identical kyanite grade meta-pelite (K98-33) from west of the snout of the Biafo Glacier, less than 5 km from T16. The discrepancy between these two ages is discussed in the main text.

Bullah dome, garnet-amphibolite orthogneiss; sample T39
This sample was a heterogeneous layered gneiss containing biotite-rich and biotite-poor layers.  The sample was separated into leucosomes and melanosomes and each dated separately. One zircon fraction (Z1) containing thirty, very small (<10 μm), needle-like crystals was picked from the melanocratic portion of this sample along with two fractions of titanite crystals. Titanite fractions T1 and T2 contained fifteen and eleven grains respectively, between 50 μm and 100 μm in size. T2 contained slightly larger crystals but most grains in both fractions were fragments. In the leucosome of this rock, pyrochlore occurs. Pyrochlore is a calcium - sodium - niobium oxide with the general formula (Ca, Na)2Nb2O6(O, OH, F). It is known to incorporate substantial amounts of radioactive elements and is related to the minerals uranopyrochlore ((U, Ca, Ce)2(Nb,Ta)2(O6(OH,F)) and calciobetafite ((Ca, REE, Th, U)2(Nb, Ta, Ti)2O7), which contain high concentrations of U and Th. Three red-brown, pleochroic pyrochlore crystals with a resinous lustre were picked to make a single analysis. Details of each fraction are shown in Table 4.2. Fraction P1 contains a very high proportion of U (13%) and Th (17%). 
Fraction P1 is nearly concordant just slightly above concordia (Fig. 8) with a 206Pb/238U age of 28.0 ± 1.7 Ma. There is scant information on the interpretation of pyrochlore ages in metamorphosed rocks, but the study of Parrish and Scammell (1988) refers to pyrochlore U-Pb ages being distinctly younger than zircon and similar to biotite K-Ar cooling ages; we prefer to interpret the 28 Ma age therefore as a time of cooling to ~ 400o C.  Fraction Z1 contained many tiny crystals and may not have been 100% pure, a possible explanation for high common Pb in this analysis.  It plots just slightly discordantly with a 206Pb/238U age of 70.1 ± 0.3 Ma (Figure S2b).  The morphology of these grains, tiny long needles is such to suppose that inheritance is likely to be absent or certainly a minor component.  The titanite data for this sample are unrevealing since the U content is very low and there is only a very minor amount of radiogenic Pb in the grains.  A two point isochron for the two titanite analyses is imprecise and gives an “age” of 82 ± 43 Ma, and therefore not useable.  
Various interpretions are possible without a more extensive dataset.  It could be reasonably argued that the protolith of the rock is ~70 Ma from the near concordant zircons that should lack inheritance, with pyrochlore giving a cooling age of ~28 Ma, in line with pyrochlore data from a few other studies in metamorphic terranes that yield similar ages to mica K-Ar or Ar-Ar dates (Parrish & Scammell 1988).  A less likely explanation might be that pyrochlore dates crystallization of leucosome with protolith age being ≥ 70 Ma, possibly Jurassic if the upper intercept of a chord from 28 Ma through the zircon analysis is used yielding 160 ± 56 Ma. There is no evidence however of an early Palaeozoic or Precambrian age to the protolith.  This unit is therefore distinct from the Dassu orthogneiss that that contains much older zircons (1860 Ma, Fraser et al, 2001; 460 Ma, this study).  Tentatively we favour a ~70 Ma protolith age of zircon crystallization and a cooling age interpretation of the 28 Ma pyrochlore data.

Dassu orthogneiss; Sample T356
Two monazite fractions were picked from this sample. M1 contained four green monazite crystals >200 μm in size. M2 contained eight, smaller (<100 μm) grains. These fractions were chosen to see if there were any differences in age between the populations of large and small monazites in this rock. Two zircon fractions were also chosen for analysis. Z1 was a large, doubly terminated, clear zircon. Z2 was a narrower crystal with a rectangular cross-section. Both crystals were approximately 200 μm in length and were air-abraded for three hours. Monazite fractions M1 and M2 plot above concordia at between 5 Ma and 6 Ma (Fig. 8) but do not overlap.  Their 207Pb/235U ages differ by ~0.5 Ma.   The significant reverse discordancy of these analyses is probably due to incorporation of excess 206Pb from the decay of 230Th (Schärer 1984), so the 207Pb/235U age is the most reliable.  208Pb/232Th ages of monazites M1 and M2 are 5.43 ± 0.09 and 5.36 ± 0.04 Ma, respectively, which are in generally good overall agreement with 207Pb/235U ages, but more consistent and we use 5.4 ± 0.1 Ma as the best estimate of monazite crystallization.  This age is the same as the U-Pb monazite age of 5.4 ± 0.2 Ma obtained by Fraser et al. (2001) and shows that there are no significant differences in crystallisation age in these samples, despite their separation in structural depth. The two zircon crystals Z1 and Z2 plot discordantly between 400 and 500 Ma. Discordia tied to the interpreted monazite age at the younger end have upper intercepts of 509 ± 2 Ma and 460 ± 2 Ma for Z1 and Z2 respectively, suggesting an early Palaeozoic age for the protolith. It appears therefore that the orthogneiss basement in this area is made up of at least two components, one emplaced at ~ 460 Ma and one represented by 1860 Ma zircons from the Dassu gneiss (Fraser et al, 2001).

Dassu aquamarine-tourmaline pegmatite; Sample T359
Zircon and monazite were separated from a spectacular pegmatite that intrudes the Dassu gneiss.  Most zircon crystals in sample T359 contain inclusions. Some crystals contain an inclusion-rich core, which may be inherited, while others contain one or two large inclusions which transect the cores. Two zircon fractions were picked and abraded from this sample, both containing a single crystal or fragment. Fraction Z1 was a long, thin zircon (~150 μm long) with a long melt inclusion parallel to its length, suggesting a lack of inheritance. Fraction Z2 is about 100 μm long and contained striations throughout. Three monazite fractions were also picked from sample T359. M1 contained two large (400-500 μm), green euhedral monazites. M2 contained five smaller (200-300 μm), euhedral to subhedral crystals. M3 contained a single, euhedral monazite crystal approximately 200 μm in size.  
Zircon fraction Z2 plots just above concordia at ~3.5 Ma (Fig. 8) whereas fraction Z1 is discordant at ~320 Ma and clearly contains an invisible inherited component, the age of which is ~1200 Ma as inferred from the 207Pb/206Pb age. The reasons for the slight reverse discordance of Z2 are unclear, but could include an incorrect assumed composition of Pb used to correct for initial Pb.  The age of Z2 is taken to be its 238U/206Pb age of 3.48 ± 0.02 Ma, though this measurement error likely underestimates the uncertainty in age of crystallisation, and indeed the crystal itself may have grown over a period of time longer than the quoted uncertainty.
The three monazite fractions (M1, M2, M3) plot above concordia between 2.5 and 5 Ma (Figure 8) in an array similar to monazite of samples T311 and T320. The reverse discordancy displayed by these analyses is due to excess 206Pb, from unsupported 230Th (Schärer 1984), and the spread of ages probably reflects an inherited monazite component or progressive crystallization of monazite from 5-2.5 Ma, though this spread could also be complicated by the very poor precision (>16% 2) on the 207Pb/206Pb ratio of M1, rendering its 207Pb/235U age unreliable. The 208Pb/232Th ages of analyses M1 and M2 are 3.64 ± 0.13 and 4.41 ± 0.10, respectively. The 3.64 ± 0.13 208Pb/232Th age of M1 is close to the measured zircon age, and we suggest that 3.5 ± 0.1 is a reasonable age of crystallization.  The fact that the ages of M2 and M3 are both older suggests that inheritance may have played a part. It is plausible that the inherited component is the host gneiss with its ~ 5.4 Ma monazites.  

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