Provenance of the Papuan Peninsula (Papua New Guinea): Zircon Inheritance from Miocene–Pliocene Volcanics and Volcaniclastics

Round 1

Reviewer 1 Report

This is a well-written and compelling paper that should contribute not only to an understanding of the regional geology, but also to terrane analysis in other parts of Earth. Since many suspect terranes across the globe continue to be debated as to whether they are allochthonous or para-autochthonous, the authors might consider pointing out these similar challenges, perhaps with examples. [Several groups are working on a similar problem with the suspect terranes of the southern Appalachian Mountains, USA, for example.] This might extend the value of the manuscript farther, and increase its readership and citation.

The introduction clearly states the objectives and sets the scene. The background section is excellent — thorough (to me), yet readable.

The methods are sound and well within the bounds of common practice, although I have some concern that the larger laser spot size used could have led to some biasing or mixing of age zones within a crystal (comment below). That isn’t a deal-breaker, but needs addressing.

I have some concerns about the description of discordance filtering, and the choice of plotting tool (i.e., the absence of KDE). These are easily fixed.

I have concern about the use of the term ‘inherited,’ which historically has inferred the incorporation of xenocrystic zircon from country rock by intruding magma. Given the description of the sampled lithologies (including mudstones, sandstones, conglomerates) I am not convinced that at least some of the older crystals are not detrital zircons — that is, grains entrained and incorporated into the sampled lithologies by sedimentary depositional systems at work at the surface. The authors need to clarify what they mean by ‘inherited’ (and if that includes detrital zircons, I suggest a different term). This is fixable. 

I (and I think many others) have concerns about the use of the ‘three-youngest’ (Y3Zo) method of interpreting a maximum depositional age, especially when those ages are the young tail of a ~Gaussian-distributed population — (e.g., the red and blue plots in Fig 5). First, n=3 is arbitrary. Second, the young tail could merely be the artifact of standard-unknown bracketing such that instrument drift can introduce apparent age differences from the same reference material that are several % apart in a matter of minutes. With a larger ‘youngest’ population of grains (e.g. Fig 5), the likelihood of a young tail goes up, yielding a Y3Zo that is substantially younger than the weighted mean age of the youngest ~Gaussian population, yielding a potentially too-young MDA. In my opinion there are several other more robust ways of extracting MDA interpretations from young populations than the arbitrary Y3Zo approach (see Vermeesch, 2021: Geoscience Frontiers for an extended discussion). Although the differences between the more robust MDA methods and the Y3Zo method used here are unlikely to have a significant impact on the results of this manuscript, because of the relative paucity of absolute ages of the studied units, they may get misused by future workers. Therefore, my recommendation is to generate MDA interpretations using a more robust approach.

I have concerns about the presentation of multiple maximum depositional ages (one ca. Pliocene, the other Cretaceous) from the same sample (e.g. Table 2). I think I understand what the authors are intending, but calling the ages of Cretaceous populations in samples that have Miocene-Pliocene zircons in them “maximum depositional ages”, or “youngest grains or clusters (i.e., YSG, Y3Zo)” is confusing and misleading at best, and arguably just wrong. If the authors are wanting to analyze the age of the older Cretaceous populations, I strongly recommend they steer clear of MDA terminology when presenting the weighted mean, etc. ages for these populations. If on the other hand they intimate that the Cretaceous populations truly are maximum depositional ages, then the  Miocene-Pliocene populations need explanation — natural (post-depositional intrusive igneous) contaminants, crystals that have suffered lead-loss (invisible because of their young age), metamorphic resetting, etc. 

It seems that the authors are assuming that all of the xenocrystic zircons (again assuming all of the older grains are xenocrysts and not detrital) in a single sample are from a single interval of sedimentary country rock along the magmatic pathway, as indicated in their interpretation of MDA from the older populations. This seems unlikely, and most definitely not certain barring a whole lot of other constraints. At the very most, all that these calculations provide is the MDA of the youngest sedimentary interval through which the magma traveled. But even this assumes both that the xenocrysts are xenocrysts and not detrital, and that they came from sedimentary rocks, in that MDA is applied to sediments and sedimentary rocks only. And even these assumptions aren't certain to be true to me.

I have major concerns about creating MDA interpretations from multiple compiled samples, as seems to be the case in Table 2 and Figure 5 and related text, wherein multiple samples from multiple levels in the same formation are lumped together and then analyzed as if they came from a single deposit. Whereas such compilations can be useful for provenance purposes, especially when multiple samples from the same unit have similar age distributions, compilation is inappropriate for determining maximum depositional ages. 

All of these problems are fixable, but they are significant and substantial. Thus I am comfortable with either accepting the manuscript pending approved major revisions, or rejecting and encouraging a resubmission. I will leave it to the editor to decide between those options. I do think the data presented are useful and important to the problem at hand and for the larger terrane analysis community, but I think some of the interpretations are based on poor assumptions. Those interpretations are not central to the main thrust of the paper, but I think they are misleading at best and could propagate some possible misunderstandings and misconceptions.

Comments tied to line numbers are listed below, with those of substance indicated with an asterisk.

11-12. oddly worded.

43-44. oddly worded: “insufficient work to establish” 

59. ‘sequences’ can be a loaded term. I prefer ‘succession’ in these contexts.

63. add ‘geology’ or ‘units’ after ‘Papuan Peninsula’?

78-79. slightly redundant and/or contradictory with prior paragraph.

87. ‘were’ instead of ‘are’?

Fig. 2. Add ‘North’ or ’N’ to North arrow

105. I think that “Late” should replace “Upper” here since the noun to which it refers is ‘age’ (rather than a stratigraphic unit, in which case Upper would be correct usage).

109. Same comment as 105.

113. oddly worded — “intruded by an episode of volcanism”. An ‘episode’ is a time period, not a physical entity so can’t intrude, per se. Also, ‘magmatism’ would be better than ‘volcanism’ since the latter is extrusive not intrusive. 

129-132. Somewhat of a run-on, cumbersome sentence here. Consider breaking into two or three?

133. I think ‘concentration’ would be better than ‘density’ here.

140. missing space

144. ‘upper’ and ‘lower’ instead of ‘late’ and ‘early’ here. Also consider deleting ‘mapped’ — it is what it is, regardless of how it is mapped.

147-149. Confusing — are the deposits fluvio-lacustrine? or shallow-marine? or both? Last phrase seems like an after-thought, and no citations nor support occur herein for these interpretations.

153. Mudstone and tuffaceous sandstone don’t just have different grain sizes, they are different lithologies altogether. 

155, 178. Same comments re: ‘mapped’ as in 144.

182. Replace ‘upper’ with ‘late’ for the noun here is a time period.

*210. 44 microns is quite a large laser spot — bigger than many zircons we recover and analyze from similar lithologies and geologic settings. Indeed several of the crystals in the CL micrographs appear to be much smaller than 44 microns. Perhaps address whether this large spot size may have had any impact on grain-size bias, or mixing of rim and core age zones?

218. typographic error here.

Figs 3. and 4. If space is needed, these figures could be moved to the supplementary materials — they are helpful but not critical for the main text.

*238-248. Clarify the discordance filtering. How were revere discordant ages treated? What if a <1 Ga grain passed one concordance filter (say 206/238 vs 207/235) but not the other (say 207/206 vs 206/238)?

*259-260. There is compelling evidence that the use of kernel density estimates (KDE) are superior to probability plots in many situations. (See the discussions of Vermeesch, etc.) — my recommendation is to include KDE as well.

*283. Don’t you mean high? Low Th/U in zircon is generally <0.1, and would be reflective of metamorphic zircon. Th/U greater than ~0.5 is typically reflective of magmatic zircon.

*308. How do you know these older crystals are inherited and not detrital? Given the depositional systems and lithologies reported in the manuscript, could some of these older grains have been incorporated through sedimentary processes? If so, I suggest that for detrital zircons, the authors stay clear of ‘inherited’ which carries the connotation of incorporation of xenocrystic zircons during magmatism. If they are truly all inherited in the conventional (xenocrystic) sense of the word, the authors will need to convince the readers and reviewers why this is so, and no later in the manuscript than here.

*324-343, Table 2. I am confused — are you reporting more than one MDA for each sample? How is that possible, unless the young populations are contaminants, victims of invisible lead-loss, or metamorphically reset (all of which seem unlikely given the regional geology)? If any of these are so, they need to be addressed prior to the presentation of Cretaceous ‘MDA’. Regardless, I strongly suggest the authors steer clear of MDA terminology for populations significantly older than the youngest grains in a given sample. It is misleading, confusing, and in my opinion, wrong.

*Figure 5. First, as indicated in my opening comments, I have strong concerns about lumping multiple samples in order to determine a unit MDA — this is misleading at best, and in my view wholly inappropriate. I strongly recommend that compiled data are not used for MDA calculations unless from the very same deposit — one wouldn’t use a younger fossil biozone to characterize the maximum depositional age of a formation that also contains older biozones lower in the unit. Second, even if the compilation problem did not exist, a more conservative interpretation would yield older MDA interpretations, especially for those wherein the weighted means of the youngest-3 (let’s be honest, n=3 is arbitrary) are the young tail of a Gaussian distribution of ages (i.e., Apinaipa and Yaifa). The nature of standard-unknown bracketing makes these MDA estimates quite possibly too young. 

358, etc. Inconsistent use of Mt Davidson versus Mount Davidson. I suggest sticking to one spelling.

*sections 6.1 and 6.2. Much of this addresses the detrital versus (truly) inherited concerns I have raised prior in the manuscript. Nonetheless, I’m still not convinced that all of the pre-Miocene zircons are xenocrysts and not detrital — additional evidence and discussion is required. Moreover, some reorganization that puts interpretation of the older zircons being inherited xenocrysts after presentation of the results could help make the paper more objective. 

*429. I haven’t yet been convinced this is true. Whereas I don’t doubt that some of the older zircons are xenocrystic, I am not certain given the sampled lithologies and depositional environments that they *all* are. The authors might want to bolster this interpretation with additional support — evidence of all other sources being overlapped (i.e., inaccessible) at the time of deposition, widespread euhedral crystals, regional paleogeographic constraints that preclude the grains being transported from afar, etc.

*449-454, Fig 7., and elsewhere. I have a real problem with interpreting an MDA of underlying country rock from a population of xenocrystic zircons. Without high-precision barometry, individual xenocrystic zircons cannot be firmly affiliated with a particular unit or interval. Therefore the interpreted xenocrystic zircons from a single sample very well could collectively represent a mixture of inherited crystals sampled from various intervals (i.e., rocks of different ages) along the magmatic path within the country rock between the magma chamber(s) and the edifice (not to mention by incorporating detrital zircons from external sources at the surface). Moreover, the xenocrystic zircons could have be entrained from igneous country rock, in which case an MDA is wholly inappropriate. 

479, 482. Typographic errors.

499. ‘basement’ usually implies crystalline rock.

501. replace ‘late’ with ‘upper’ since the modified noun is ‘rocks’

504-519. This is quite a long laundry list and run-on sentence, and seems redundant with information contained in Fig 8.

552, 556, etc. I think the plural of spectrum is spectra?

*section 6.3. It might be due to my lesser familiarity with the regional geology, but in contrast to the rest of the very well-written paper the writing here gets quite difficult to follow, and seems a bit ‘in the weeds’ as we say in the US. Could it be reduced or simplified? If these matters are of importance it might be useful to put the main points here and relegate the details to the supplementary materials. At the least however, each point should have its own paragraph. 

*679 Again my concerns with the accuracy of all older grains being inherited.

Author Response

The authors would like to thank the reviewer for their detailed review and comments. Please see attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

This study utilizes zircon U-Pb geochronological results to refine the tectonic history of the southeastern reaches of the Papuan Penninsula. The paper uses standard and well accepted methodology (U-Pb zircon dating) and provides adequate documentation of the analytical results. I see no major issues with the approach, analytical methodologies, and/or resulting interpretations. While it is possible that someone with greater up-to-date knowledge regarding the tectonic history of the northern Australian margin may have some criticisms of the author's interpretations, everything seems well supported to me. I believe that the paper will make an important contribution to the growing body of of knowledge supporting interpretations of the region's tectonic history. I recommend publication. 

Author Response

The reviewer is thanked for their review and comments. The authors note that no specific corrections are required by the reviewer.

Reviewer 3 Report

I found this manuscript to be a joy to read and found the conclusions particularly interesting. I think that you have made a significant new contribution to an understanding of the Papuan Peninsula. I hope that there is more to come.

Author Response

The reviewer is thanked for their review and comments. The authors note that no specific corrections are required by the reviewer.

Reviewer 4 Report

Dear authors,

It was a pleasure reading about your latest results from the Papuan Peninsula. The manuscript is very well written and the figures/graphics are excellent. I have attached an annotated PDF that contains some very minor amendments/corrections to the text.

Comments for author File: Comments.pdf

The manuscript is very well written. The attached annotated PDF highlights some very minor amendments/corrections.

Author Response

The reviewer is thanked for their review and comments. All minor revisions have been made as suggested.

Round 2

Reviewer 1 Report

The authors made or convincingly addressed most if not all of the minor suggested corrections from the prior report.

Some of the major suggested corrections were made, or at least partially addressed. Thank you.

Other major suggested corrections were not made or sufficiently addressed in my view:

• The authors claim a minimization of analytical bias by random selection of zircons, yet their used spot size seems to have precluded the analysis of small zircons as indicated by the crystals that were and were not analyzed in the supplementary materials CL imagery. Let me clarify by saying that this is a necessary evil that we all face as we try to maximize signal to acquire the lowest uncertainty data possible, and therefore the lack of data from small or age-zoned crystals should not get in the way of publication. However, I think it is important to acknowledge a possible analytical bias when smaller or age-zoned populations were not analyzed with the frequency of larger crystals. Transparency is important, especially when one is trying to establish MDA. My concern would be addressed or removed if the authors included a brief statement in the manuscript addressing the fact that some smaller zircons were preferentially not analyzed due to spot size limitations, which may incorporate an (unavoidable using the employed methods) source of bias into the data. Again, this analytical bias does *not* preclude publication, in my view — the data are valuable regardless of analytical bias. But the data aren’t free of possible bias as the manuscript implies. This is of particular importance for constraining the age of eruption/deposition because first-cycle volcanogenic zircons that are closest to the eruptive age are likely to be from the smaller populations given the shorter crystallization times during volcanism.
• The authors insist upon basing their MDA interpretations on an arbitrary method (‘Y3Zo’ adapted originally from Dickinson & Gehrels, 2009: EPSL) that has been undermined in the more recent literature (e.g., Vermeesch, 2021: Geoscience Frontiers), and did not provide a convincing rebuttal to my criticism other than to state that ‘numerous published studies globally’ use the method. Just because something is or has been done, and is in print doesn’t mean it is correct, or the best practice. The authors seem to be discarding the results of the more robust method because it doesn’t fit their narrative, at least for the Cretaceous populations? But, perhaps that is because MDA approaches are not appropriate for inherited populations (see following comments).
• I’m still troubled by the use of MDA and MLA techniques on the older populations of inherited zircons. In my view such approaches should only be used on discrete sampled units, not a composite of crystals likely entrained from different levels by (uncertain degrees of) magmatic and detrital processes. All that one knows from such composite age data is one *possible* maximum depositional age from the *youngest* zircon-bearing unit through which the magmatic and/or sedimentary systems passed, *and* that sampled zircons from it. Allow me an example: say 90% of the geology through which the magmatic and sedimentary systems passed was composed of (zircon-bearing) lower Paleozoic or older units, and 10% was composed of Cretaceous-age rocks, a Cretaceous ‘MDA’ of the composite zircon population entrained through these processes is only relevant for the youngest unit that was sampled by the magmatic and/or sedimentary systems. That is, it would *not* be relevant for 90% of the geology through which the systems travelled and entrained ‘inherited’ zircon. Whereas the authors likely understand this, the nuances aren’t addressed in the manuscript, and I fear misconceptions will be propagated by less familiar readers — as the authors note with the Y3Zo method, once approaches make it into print — however inferior or even erroneous — subsequent readers assume that such approaches are valid, even when better approaches emerge. 
• Related: in their rebuttal, the authors imply that the Owen Stanley Metamorphic Complex is the sole source for the inherited grains. Perhaps it is because of my lesser familiarity with the regional geology, but with mantle-derived magmas and crustal thicknesses of up to 35 km on the Papuan Peninsula (Abers et al. 2002: Nature), I find it difficult to accept that the entirety of the crust through which the magma travelled is a single geologic unit. If this (unlikely?) scenario is indeed so, the authors need to clarify or defend with geophysical or geologic data and arguments. Is there evidence that the Owen Stanley composes all of the zircon-bearing subsurface rocks down to the Moho? Barring such evidence I find it difficult to accept an interpretation that all inherited grains come from the Owen Stanley.
• I don’t see the value of a “formational maximum depositional age” and fear that it is misleading at best, and maybe misinterpretive. By definition, a maximum depositional age relies upon the principle of inclusion — a deposit cannot be older than the age of the youngest detritus within the deposit. Therefore a “formational maximum depositional age” only pertains to the youngest part of the formation sampled by the inheriting magma (or sedimentary system). I fear that the authors are implying (or that others will interpret them as doing so) that the entirety of the Owens Stanley complex (*even if it were the only unit to have contributed inherited zircons to the sampled units*) is no older than ca. 77-104 Ma, when in reality parts of it could be much older. Indeed, back of the envelope calculations indicate that the Y3Zo-based MDA of sample 103259 of the Mt Davidson volcanics is ca. 417 Ma, that of sample 103257 of the Yaifa Formation is ca 418 Ma, that of samples 103271 and 103273 of the Kore volcanics is ca. 438 and 402 Ma, respectively; and the youngest pre-Cenozoic crystal in sample 103258 of the Yaifa Formation is ca. 172 Ma (!). These are stark examples of how composite “formational MDA” and MDA interpretations from inherited zircons could be fraught with problems. In addition to my concerns raised above, they imply that (1) the intruding magmas incorporated inherited zircons from units other than the Owens Stanley complex, and/or (2) the MDA of the Owens Stanley complex isn’t everywhere mid-Cretaceous, and/or (3) the Y3Zo method (especially for small-n samples) is inappropriate… I find all three of these possibilities to be compelling, and cause for cautious interpretation.

In sum, the authors did not sufficiently address many of my original concerns with the manuscript or in their rebuttal. I stand by the belief that there are important and valuable contributions within the data and the manuscript. But without a more thorough and careful handling of these concerns, the manuscript will need to be published without my endorsement. Given the positive reviews of the other reviewers, the Editors may wish to find an additional reviewer to assess whether my criticisms are justified. I do feel strongly that the misapplication of MDA to older inherited zircon populations is scientifically unsound. 

Author Response

The authors would like to thank the reviewer for their second review and comments. The major comments are summarized and addressed below:

1) Bias of zircon grains analyzed due to laser spot size:

• Additional commentary has been added to the methods together with the laser spot size, in line with the reviewers suggestion:
• "Some smaller zircon grains were not analyzed due to the spot size, which may incorporate a source of bias into the data. First-cycle volcanogenic zircons that are closest to the eruptive age are likely to be from the smaller populations given the shorter crystallization times during volcanism."

2) Use of MDA or MLA methods for interpretation:

• We have emphasised that the MLA method is considered the most statistically robust method, however, other methods should not be discounted entirely. It is likely another new method will be subsequently introduced in the future, which will not discount the MLA method presented by Vermeesch.
• We present all methods to calculate the maximum depositional age with their results - and understand that the depositional age may be younger than the analytical result.
• We consider the geological context together with the analytical results and provide our interpretation of the most geologically reasonable results, taken together with previous work from prior mapping relationships and biostratigraphy from other authors. The selected methods produce, in our opinion, appropriate results given the known understood geological context of these samples. 

3) Potential for sampling of zircon grains from unknown units:

• Yes, we acknowledge it is possible that other units may has been sampled as magmas migrate through the crust, but given our current knowledge there is no evidence for older continental material beneath the Papuan Peninsula. This understanding may change with more work in the future but given the lack of current evidence, and the abraded nature of the older zircon grains, we have interpreted them as detrital grains.
• An additional sentence has been added to the section discussing the origin of the inherited zircons, "This does not preclude the existence of older continental basement beneath the Owen Stanely Metamorphic Complex, however, there is currently no direct evidence for such older crust [3,5,8,21,23]."
• The over-thickened crust in this area, in my opinion, is more likely the result of recent Miocene subduction, collision of a crustal plateau fragment from the south, more recent underthrusting of the oceanic Solomon Sea Plate to the north, and shortening of the crust, rather than an isostactic-stable stack of continental crust.

4) Formational maximum depositional ages over unit-specific ages:

• The academic knowledge of Papua New Guinea could be considered to be at the foundational stage compared with many developed countries. The Owen Stanley Complex, while very much a regional grouped unit, has not had sufficient research to subdivide this beyond what is outlined in the geological setting.
• Given the above, yes there is significant scope for further research that could further refine and/or revise our understanding of the regional stratigraphic units, but this study cannot and does not attempt to solve all of these geological questions.
• Our results and interpretations are given in the context of the level of knowledge (and limitations) of Papua New Guinea geology. At this level, it would be inappropriate to claim to be able to subdivide the Owen Stanley Complex beyond a regional unit.
• Given these limitations and the above comments to the reviewers other queries (e.g. nature of the crust beneath the Owen Stanley Complex), the authors consider the level of the interpretation is appropriate for the context and framework for the study area.

We hope this addresses the reviewers concerns and provides more context for Papua New Guinea geology and this study. This is a complicated area without a track record of detailed academic studies. Our understanding of the geology is still at the foundational stage - this study outlines that we don't even know where the terrains formed - and this region may be of interest for further scientific investigation to address the many unanswered questions, some of which are raised by the reviewer.